Fair Question to be Tried and Expert Causation Evidence in Environmental Health Injunctions: Analysis of M.A. & Anor v M.C. & Ors [2025] IEHC 618

1. Introduction

This judgment of Ms Justice Emily Farrell in the High Court (Planning & Environment list), M.A. & Anor v M.C. & Ors [2025] IEHC 618, addresses an increasingly sensitive intersection: alleged health impacts of agricultural chemicals and the law of interlocutory injunctions.

The plaintiffs, a couple living in rural Connacht with their 15-year-old daughter, sought orders effectively creating a 100-metre buffer zone around their home, within which neighbouring farmers (the first three defendants) would be prohibited from using pesticides or spreading slurry. They also sought to restrain a range of alleged antisocial and harassing behaviours. Their case was framed in dramatic terms: “disguised chemical attacks”, “gas poisoning”, and assertions that without an immediate injunction their deaths were likely.

Crucially, the defendants’ farming practices involved products lawfully authorised under Irish and EU law, specifically the regime implemented by S.I. No. 155 of 2012 (European Communities (Sustainable Use of Pesticides) Regulations 2012). The plaintiffs did not challenge the legality of that regime; instead, they alleged that the defendants’ lawful use nevertheless violated their constitutional rights to life, bodily integrity and their child’s safety, and caused personal injuries and property damage.

The application before the Court was interlocutory: the plaintiffs sought interim injunctive relief pending trial, despite labelling their motion as seeking a “PROHIBITIVE PERMANENT INJUNCTION”. The fourth and fifth defendants (a Department of Agriculture official and the Minister) were not the subject of the interlocutory application.

The decision is significant because it:

• Clarifies the level and type of evidence required even to cross the relatively low “fair question to be tried” threshold for interlocutory injunctive relief in environmental health disputes.
• Reaffirms that courts will not speculate on central elements of a plaintiff’s case – in particular, on scientific causation – without at least some cogent medical or expert support.
• Emphasises that lay litigants are bound by the same rules of pleading and evidence as represented parties, especially regarding hearsay and opinion evidence.
• Demonstrates judicial deference to the EU/Ireland regulatory framework for pesticide authorisation when those regulations are not directly challenged.

2. Summary of the Judgment

2.1 Procedural Posture and Claims

The plaintiffs issued a plenary summons on 30 June 2025. The general endorsement of claim was unusual: it purported (a) to “prosecute” neighbours under the Offences Against the Person Act 1861 and the Criminal Damage Act 1991, and (b) to hold the Minister and a departmental officer responsible for allegedly allowing “uncontrolled and unlimited” pesticide use causing personal injury and property damage.

No properly pleaded statement of claim had been served. A lengthy document labelled “Statement of Claim” was uploaded to ShareFile but not delivered to the defendants and did not comply with Orders 19 and 20 RSC. It mixed legal claims with policy and advocacy headings such as “End of Social Injustice” and “Judicial Review Resolving Dysfunctional Legislation”.

The motion dated 15 July 2025 sought, inter alia:

“a PROHIBITIVE PERMANENT INJUNCTION stopping the administration of Environmental Poisons in form of pesticides and other agricultural toxins… at our Home address… in order to save our Lives immediately and IN URGENCY…”

In practice, by the time of the hearing, the core relief sought was an injunction prohibiting the first and second defendants from using pesticides within 100 metres of the plaintiffs’ property (a “buffer zone”), and an order restraining alleged harassment and damaging behaviour. No relief was sought against the fourth or fifth defendant at this stage, and there was doubt whether the third defendant had even been served.

2.2 Evidence Before the Court

Two affidavits were sworn by the plaintiffs. The first and second defendants each filed replying affidavits, to which the plaintiffs responded.

• The plaintiffs described a range of medical symptoms they and their daughter allegedly suffered, and attributed them to pesticide exposure (and, in the third defendant’s case, to slurry spreading).
• They exhibited a veterinary report for a cat that died of kidney failure, but which did not identify pesticide poisoning as the cause.
• They provided no medical reports, GP letters, or expert opinions linking their ailments (or any of the alleged animal or tree damage) to pesticide use.
• They included speculative and intrusive assertions about the health and causes of death of neighbours and relatives of the defendants, supported only by death notices from rip.ie and their own suppositions.
• They asserted that laboratories in Ireland had declined to carry out the tests they wanted, but acknowledged that they could, in principle, pay a German laboratory for such testing.

The defendants, by contrast, averred:

• They use pesticides lawfully, in limited and controlled quantities.
• Their practices are inspected by the Department of Agriculture and by Bord Bia, and all inspections had been passed.
• The plaintiffs’ allegations of overuse and malicious “chemical attacks” were untrue and deeply upsetting.

The second defendant offered an undertaking intended to reassure the plaintiffs, but they refused, indicating instead that they were holding their own “Private Court”.

2.3 The Court’s Key Findings

1. No “fair question to be tried” established on the core allegation of pesticide-related harm.
   The Court accepted, for the limited purposes of the interlocutory application, that the plaintiffs and their daughter suffer the medical conditions they describe (para. 30). However, there was:
   • No expert medical or scientific evidence.
   • No admissible evidence creating even a prima facie causal link between pesticide or slurry use and the alleged injuries.
   • A heavy reliance on speculation, conjecture and personal belief (para. 37).
   In those circumstances, the plaintiffs had not discharged the burden of proof even at the low threshold required to show that there is a “fair question to be tried” (paras. 40, 47).
2. Court will not speculate on central elements of a claim.
   Citing Nihill v Everyday Finance DAC and Rogers v Allied Irish Banks, Farrell J held that a court cannot speculate about central parts of a plaintiff’s case (here, causation), and that evidential gaps cannot be filled by inference or by reliance on findings from other cases such as John Donnelly & Sons Ltd v Hoey (paras. 40, 45).
3. Deference to the pesticide regulatory regime.
   The plaintiffs accepted that the pesticides in question were authorised for use and that S.I. 155/2012 (implementing Directive 2009/128/EC) was valid (para. 10). The Court emphasised:
   • There is a presumption of validity in respect of that statutory instrument (para. 41).
   • There was no evidence before the Court contradicting the Department’s statement that products are only authorised if they are shown to have “no unacceptable impacts on human and animal health and the environment” (para. 42).
   In light of this, the Court could not accept the plaintiffs’ theory that lawful use per se created a presumptive health risk justifying a buffer-zone injunction.
4. Improper and speculative affidavit material struck out and given no weight.
   The judge struck out averments about the health of the defendants’ wives and rejected reliance on death notices as a basis for imputing pesticide-related causes of death (paras. 33–34). Such material was “entirely inappropriate”, offensive and speculative, and significantly undermined the weight that could be placed on the plaintiffs’ affidavits (paras. 34, 37, 49).
5. No fair issue for an injunction concerning alleged harassment.
   The plaintiffs also sought orders restraining swearing, death threats, harassment and damage to trees. All such behaviour was denied. The judge accepted that if such conduct occurred it would be inappropriate and might be criminal, but on the totality of the evidence she was not satisfied that a fair issue to be tried was established so as to justify injunctive relief against the first and second defendants (para. 49).
6. No need to address balance of convenience or undertaking as to damages.
   Because the plaintiffs failed at the first stage (no fair question to be tried), the Court did not proceed to analyse the balance of convenience / balance of justice or adequacy of damages (para. 50). The plaintiffs had not proffered an undertaking as to damages (para. 51), but that was not determinative.
7. Limits on using civil proceedings to “prosecute” criminal offences.
   The Court held that:
   • A litigant cannot institute criminal proceedings in the High Court by issuing a plenary summons (para. 24).
   • The specific 1861 Act provisions relied upon (ss. 23–24) had in any event been repealed by the Non-Fatal Offences Against the Person Act 1997 (para. 24).
   • Accordingly, no fair question to be tried arose on those elements of the claim.
8. Outcome and costs.
   The interlocutory injunction application was dismissed (para. 53). Costs were awarded to:
   • The first defendant (full costs of the motion, including written submissions).
   • The second defendant (legitimated expenses and outlay) (para. 54).
   However, importantly, there was a stay on the adjudication and execution of those costs pending the determination of the plenary proceedings (para. 55). The Court refused the first defendant’s application to stay the proceedings until costs were paid.

(References in parentheses are to paragraph numbers in the judgment.)

3. Analysis

3.1 Precedents and Legal Materials Cited

3.1.1 Merck Sharpe & Dohme Corporation v Clonmel Healthcare Ltd [2020] 1 I.R. 1; [2019] IESC 65

Farrell J’s framework for analysing the interlocutory injunction is firmly anchored in O’Donnell J’s (as he then was) seminal Supreme Court judgment in Merck v Clonmel (para. 19).

Key points derived and applied:

• The “logic of an interlocutory application” is that it is determined in advance of the full trial, on affidavit evidence, and the Court should not attempt to predict the full outcome on the balance of probabilities.
• Instead, the Court should ask:
  1. Is there a “fair question to be tried” – i.e. is the claim neither frivolous nor vexatious?
  2. If so, how should matters be regulated pending trial, applying the balance of convenience/balance of justice, including adequacy of damages?
• This is a restatement and refinement of the earlier Campus Oil / American Cyanamid approach, emphasising flexibility and a pragmatic focus on minimising the risk of injustice (para. 20).

In this case, the plaintiffs failed at stage (1). The Court accepted that, in principle, a final injunction could be granted at trial to restrain neighbours from activities causing personal injury, even if otherwise lawful (para. 27). But it held there was simply insufficient evidence to show that the plaintiffs had any realistic prospect of establishing causation at trial. That conclusion is grounded squarely in Merck v Clonmel’s insistence that the “fair question to be tried” test is low, but not illusory.

3.1.2 Campus Oil v Minister for Industry and Energy [1983] I.R. 82; [1983] IESC 2

Campus Oil introduced into Irish law the American Cyanamid style test for interlocutory injunctions. Farrell J reiterates, following Merck v Clonmel, that the key contribution of Campus Oil was to “reassert the flexibility of the remedy” and to focus on crafting a “just solution pending the hearing” (para. 20).

In the present case, however, the Court never reaches the balancing stage; the failure to establish a fair question to be tried stops the analysis at the gateway.

3.1.3 Fennell v Slevin [2023] IECA 177 (Edwards J) – Hearsay at Interlocutory Stage

The Court recalls the appellate guidance in Fennell v Slevin: hearsay can be admitted at interlocutory stage, but with conditions (para. 17):

• The person who made the statement must be identified.
• The party relying on hearsay must state that they believe the hearsay statements to be true.

This matters because much of the plaintiffs’ affidavits consisted of:

• Unattributed statements (“we were told”, “it is known that…”).
• Speculation about causes of illness and death of third parties.

While Farrell J does not exclude all hearsay outright, she effectively gives it little or no weight where these minimum safeguards are lacking and where the content strays into expert opinion territory without expertise.

3.1.4 Opinion Evidence and Expert Testimony

The Court restates the basic rule that opinion evidence is inadmissible unless given by an appropriately qualified expert (para. 18). Two corollaries follow:

• The plaintiffs, who are not medical or scientific experts, cannot give expert opinions on causation of disease by pesticide exposure.
• Even if they had relevant professional qualifications, they could not act as their own experts in support of their own case.

This dovetails with the more general common law requirement in toxic tort / environmental health litigation: to establish a causal link between exposure and injury, a plaintiff almost always needs medical/scientific expert evidence. That requirement applies even at interlocutory stage if causation is a central plank of the “fair question to be tried” analysis.

3.1.5 Nihill v Everyday Finance DAC [2022] IEHC 484; Rogers & Anor v AIB plc & Ors [2024] IEHC 50

While arising in different factual contexts, Farrell J draws on Dignam J’s warning in both cases: courts cannot speculate on central parts of a plaintiff’s case (para. 40). Where there is a fundamental evidential gap, a claim cannot survive even the relatively light “fair question” screening.

In this case, the “central part” of the plaintiffs’ case is the alleged causal link between:

• Lawful pesticide (and slurry) use by the defendants, and
• The plaintiffs’ and their daughter’s health conditions, pet deaths, and tree damage.

With no expert reports, no medical letters attesting to pesticide-related injury, and only the plaintiffs’ beliefs and photographs, the Court declined to bridge the gap by speculation.

3.1.6 John Donnelly & Sons Ltd v Hoey & Anor [2024] IEHC 52

The plaintiffs relied on Donnelly, where Barr J awarded damages for crop damage (speckled apples) caused by spray drift of herbicide “Spotlight Plus” from the defendants’ potato field.

Farrell J distinguishes that case (para. 45):

• Donnelly concerned proven property damage (apple speckling), not alleged personal injury.
• It was decided on the basis of a full evidential record, including expert evidence, at trial.
• Findings of fact in that case cannot be transposed to fill “evidential lacunae” in this very different case.

The judgment thus underscores a key methodological principle: fact-finding in one case cannot substitute for evidence in another, particularly when the subject-matter and evidential foundation differ.

3.1.7 Malone & Ors v Laois County Council & Ors [2025] IEHC 345

Farrell J cites Holland J’s observation in Malone that:

“ultimately and essentially the same rules apply to lay litigants as apply to represented litigants. Fairness to both sides is the watchword.” (para. 13)

This is applied here to justify:

• Insisting on compliant pleadings and service.
• Enforcing evidential rules on hearsay and opinion evidence.
• Rejecting attempts to conduct a “Private Court” or to recast the High Court’s role.

The plaintiffs’ self-representation did not entitle them to a lower standard of proof or an informal suspension of procedural norms.

3.1.8 Statutory and Constitutional Framework

• S.I. No. 155 of 2012 (Sustainable Use of Pesticides Regulations)
  Implementing Directive 2009/128/EC, this SI sets a national framework for pesticide use. The plaintiffs accept its validity (para. 10), and the Court emphasises its presumption of legality and its stated objective of ensuring no “unacceptable impacts” on health and environment (paras. 41–42).
• Offences Against the Person Act 1861 & Non-Fatal Offences Against the Person Act 1997
  The plaintiffs purported to “prosecute” neighbours under ss. 23–24 of the 1861 Act. Farrell J notes that:
  • Civil proceedings cannot be used to institute criminal prosecutions in the High Court.
  • Sections 23–24 have been repealed by the 1997 Act (para. 24).
  • Thus, no arguable claim arises on this limb.
• Injuries Resolution Board (previously PIAB)
  The plaintiffs mention that their GP declined to complete forms needed to apply to the IRB (para. 26). Technically, claims for personal injuries in Ireland generally require prior authorisation from the IRB. However:
  • The defendants did not take a procedural point on the lack of authorisation.
  • The Court therefore did not base its decision on this issue (para. 26).
• Constitutional provisions (Articles 40.3.2 and 42A)
  The plaintiffs framed their motion as enforcing their constitutional rights to life, bodily integrity and their child’s safety at home. Farrell J implicitly accepts that such rights exist and may be vindicated through appropriate relief, but holds that even where constitutional rights are invoked, the same evidential threshold for interlocutory injunctions applies: a plaintiff must still show at least some credible material supporting an arguable case of rights infringement.

3.2 The Court’s Legal Reasoning

3.2.1 Framing the Justiciable Issues

Farrell J first had to determine whether the reliefs claimed in the plenary summons and interlocutory motion were justiciable at all (paras. 21–27):

• Criminal “prosecutions” under the 1861 Act and 1991 Act: not justiciable in civil plenary proceedings; relevant provisions repealed; no fair question arises (para. 24).
• Complaints against the Minister and authorised officer (fourth and fifth defendants): these relate to alleged abuse of authority in regulating pesticide use, but no interlocutory relief is presently sought against them (paras. 4, 22, 24).
• Personal injuries and constitutional rights claims against neighbouring farmers: these provide a potential basis for injunctive and compensatory relief at trial, and thus can ground an interlocutory application if the evidential threshold is met (para. 27).

The Court, therefore, focused on the civil and constitutional dimensions of the claims against the first and second defendants: alleged health risks and injuries arising from their use of pesticides, and, tangentially, their alleged harassment.

3.2.2 The “Fair Question to be Tried” Analysis

The core of the reasoning lies in the application of the “fair question to be tried” test to the plaintiffs’ environmental health claims.

Farrell J accepts arguendo:

• The plaintiffs genuinely believe they are being poisoned.
• They and their daughter have the medical symptoms they describe (for the purposes of the motion).

But she distinguishes belief in causation from evidence of causation:

• No medical records or GP notes were produced to support the alleged diagnoses (para. 16).
• No expert report linked the alleged symptoms to pesticide exposure (para. 15).
• The only document from a health professional was an appointment letter.
• The veterinary report on the cat did not attribute its kidney failure to pesticides (para. 16).

The plaintiffs’ affidavits instead:

• Conflate correlation (e.g. proximity to spray events) with causation.
• Speculate about “gas poisoning” and “heart attack” without any medical consultation (para. 30).
• Imply that high cancer rates nationally “suggest” a pesticide-related public health crisis and that their case “confirms” it (para. 37).

Farrell J describes the plaintiffs’ case as based on “belief and suspicion” (para. 37), with allegations that the defendants’ use of pesticides constitutes a “disguised chemical attack” coordinated to harm them (paras. 9, 37–38).

Having regard to:

• The absence of medical/scientific evidence.
• The lawful authorisation of the pesticides used.
• The presumption of validity of the regulatory regime.
• The speculative, often inappropriate, content of the plaintiffs’ affidavits.

the Court held that there was insufficient evidence to demonstrate even a prospect that the plaintiffs could establish causation at trial (paras. 40, 47). That is the critical finding on the “fair question” test.

3.2.3 Respect for the Regulatory Regime and Its Evidential Consequences

The plaintiffs accepted that:

• The substances used were lawfully purchased and authorised for use.
• Use on the road verge was permitted under S.I. 155/2012 (para. 10).

The Court noted the Department’s explanation that pesticide active substances are approved at EU level, then nationally authorised only if they are effective and safe with no “unacceptable impacts on human and animal health and the environment” (paras. 10, 42).

Farrell J did not treat this as conclusive proof that no individual could ever be harmed. However, she did treat it as:

• A starting point – a legal and scientific backdrop suggesting that, absent evidence to the contrary, authorised uses are not presumed dangerous.
• A reason why the Court should be particularly slow to infer serious health risks from lawful use in the absence of expert evidence.

In effect, the regulatory regime raises the evidential bar the plaintiffs must clear: if they wish to contend that authorised pesticides, used lawfully, are in fact harming them, they must bring credible, case-specific expert evidence rather than rely on general fears or policy objections.

3.2.4 Treatment of Speculative and Intrusive Allegations

The plaintiffs made disturbing assertions about:

• The health of the defendants’ wives.
• The causes of death of neighbours, inferred from online death notices.

Farrell J characterised these averments as:

• Speculative and unsupported by identifiable sources (para. 33).
• “Extremely upsetting, distressing and offensive” (para. 34).
• Entirely inappropriate to exhibit in affidavit form.

She struck out the averments about the wives’ health (para. 34) and placed no weight on the death-notice-based speculation (para. 33).

Importantly, the plaintiffs attempted to invert the burden of proof by demanding that the defendants produce “genuine and truthful medical proof” that their families are not suffering pesticide-related illnesses (para. 34). The Court firmly reminded them:

• The burden of proof lies on the plaintiffs to establish an arguable case (para. 39).
• It is not for the defendants to disprove conjectural allegations about their own health.

This aspect of the judgment is a cautionary tale about:

• The limits of what may appropriately be put into affidavit evidence.
• The dangers of ad hominem speculation in civil litigation.
• The importance of maintaining respect for the privacy and dignity of third parties.

3.2.5 Harassment and “Social Disorder” Elements

Beyond the environmental claims, the plaintiffs sought injunctions to restrain:

• Swearing and shouting.
• Verbal assaults and death threats.
• Being asked to leave their property.
• Retaliation or harassment, including alleged stalking and damage to trees (para. 8).

These allegations were fully denied by the first and second defendants (para. 48).

Farrell J acknowledges that such behaviour, if proven, “would be inappropriate, and it may well amount to a criminal offence” (para. 49). However:

• On the totality of the affidavits, she was not satisfied that a fair issue was made out justifying injunctive orders against the first and second defendants (para. 49).
• Given the earlier concerns about the speculative nature of many of the plaintiffs’ assertions, she was not prepared to grant far-reaching behavioural injunctions on such a contested factual basis.

This underscores that anti-harassment injunctions still require:

• Clear evidence of a pattern of conduct,
• Showing that there is a real risk of repetition and that damages would be inadequate.

Those elements were not established here.

3.2.6 The Role of the Court vs. “Private Court”

The plaintiffs’ response to the second defendant’s undertaking referred to their holding a “Private Court” and demanding conditions for removing him from the case (para. 12).

Farrell J reiterates the Court’s constitutional role (para. 13):

• The High Court’s function is to independently and impartially determine the issues according to law.
• It is not to “descend into the proceedings”, take over case management for either party, or act as adviser.
• Lay litigants must comply with the same rules as represented parties, in accordance with Malone v Laois CC.

This is a gentle but firm pushback against attempts by parties (particularly unrepresented ones) to personalise the litigation, or to treat the Court as another forum in an ongoing neighbourhood dispute rather than a legal institution bound by evidence and procedure.

3.2.7 Costs and Access to Justice

While awarding costs against the plaintiffs for the motion (para. 54), Farrell J:

• Stayed adjudication and execution of those costs until after the conclusion of the proceedings (para. 55).
• Refused the first defendant’s application to stay the proceedings until the plaintiffs discharged the costs (para. 55).

This strikes a balance between:

• Protecting successful defendants from unrecoverable expense.
• Preserving the plaintiffs’ access to the court system to pursue their remaining claims at trial, where they might yet, in theory, marshal proper evidence (para. 52).

The Court also flagged that if any party contests the proposed costs orders unsuccessfully, further costs orders may follow under O. 103 r. 40(4) RSC (para. 56) – an incentive to avoid unnecessary satellite litigation over costs.

3.3 Impact and Future Significance

3.3.1 Environmental and Toxic-Tort Litigation

The decision is especially important for litigants seeking to control neighbours’ use of pesticides, slurry or other agricultural inputs through civil injunctions. It sends a clear message:

• Authorised, lawful use of pesticides cannot be treated as inherently suspect. If a plaintiff asserts serious health risks arising from such use, they must provide medical and/or scientific evidence taking issue with that general presumption or showing that the specific use, dosage, or circumstances are dangerous.
• Central causation issues must be supported by expert evidence even at interlocutory stage. The “fair question to be tried” threshold in environmental health disputes will ordinarily require at least a preliminary expert opinion linking the alleged exposures and harm.
• Courts will be slow to create de facto regulatory zones (e.g. 100m buffers) by private injunctions when the national and EU legislator has already set a regulatory framework and the products are authorised under that system.

For practitioners, the lesson is blunt: do not commence or argue an injunction application alleging health impacts from regulated substances without expert support. At minimum, a short preliminary expert report or GP letter indicating a plausible exposure–injury link is essential.

3.3.2 Lay Litigants and Evidential Discipline

The judgment is also a significant statement on the expectations placed on lay litigants:

• The courts are willing to consider serious allegations from lay litigants, including those alleging threats to life and health.
• However, such litigants are not exempt from rules on:
  • Pleadings (clarity, conciseness, service).
  • Evidence (no speculative opinion; proper treatment of hearsay).
  • Relevance and propriety (no unfounded allegations about third parties’ health or causes of death).

Judges are increasingly explicit, as Farrell J is here, that courts will:

• Strike out scandalous or oppressive affidavit material.
• Give little weight to affidavits replete with conjecture and unsupported assertions.
• Reject attempts to shift the burden of proof onto defendants to disprove vague accusations.

3.3.3 Constitutional Rights and Private Environmental Disputes

The plaintiffs framed their case in constitutional terms, invoking Articles 40.3.2 and 42A. While the judgment is not a constitutional law treatise, it implicitly reaffirms that:

• Constitutional rights can, in principle, be engaged by environmental conditions that threaten life or bodily integrity.
• However, invoking constitutional language does not relax the evidential requirements for interim relief. The same “fair question” and causation principles apply.
• Private disputes over environmental impacts must be litigated with proper evidential support; they cannot bypass the need for proof by simply invoking high-level rights language.

3.3.4 Regulatory Deference and Collateral Attacks

Although the plaintiffs did not formally challenge S.I. 155/2012 or the EU approvals process, their narrative effectively sought to treat pesticide use authorised under that framework as a de facto wrong. The Court’s approach signals:

• Where the legislature and regulators have enacted a detailed framework with explicit safety findings, courts will assume its validity and reliability unless cogent evidence to the contrary is produced or a direct legal challenge is mounted.
• Private nuisance or personal injury claims cannot be used to mount a backdoor, purely factual attack on that framework without engaging with it in legal terms and producing expert evidence.

3.3.5 Costs, Risk and Access to Justice

The decision on costs – awarding them against the plaintiffs but staying their enforcement – reflects a nuanced approach:

• Discouraging unmeritorious or speculative interim applications that impose significant expense on neighbours.
• Yet avoiding the draconian step of effectively shutting the plaintiffs out of access to a trial by requiring them first to discharge interlocutory costs.

Future litigants in environmental and health disputes should be acutely aware that unsuccessful interlocutory motions can carry real costs consequences, even where they are lay litigants and even if their underlying concerns are sincerely held.

4. Clarifying Complex Legal Concepts

4.1 Interlocutory Injunction

An interlocutory injunction is a temporary court order made before the final trial, designed to regulate the position between the parties pending a full hearing. In this case, the plaintiffs wanted an interlocutory order that would immediately prevent their neighbours from using pesticides or slurry within 100 metres of their home, and restrain certain behaviours, until the plenary action was finally decided.

4.2 “Fair Question to be Tried”

To obtain an interlocutory injunction, a plaintiff must show there is a fair question to be tried – that the claim is not frivolous or vexatious and that, based on the current evidence, there is a real, as opposed to a fanciful, prospect of success at trial. It is a low threshold, but it is not a rubber stamp: where a central element of the claim (like causation) is entirely unsupported by evidence, the test is not met.

4.3 Balance of Convenience / Balance of Justice

If there is a fair question to be tried, the court next considers the balance of convenience or balance of justice:

• Which course of action (granting or refusing the injunction) carries the greater risk of injustice if the final outcome turns out to be different?
• Are damages an adequate remedy if one side is wrongly restrained or not restrained?
• What are the practical consequences for each party?

Because Farrell J found no fair question to be tried, she did not reach this stage.

4.4 Undertaking as to Damages

An undertaking as to damages is a promise by the plaintiff that, if they obtain an interlocutory injunction and it later turns out the injunction should not have been granted, they will compensate the defendant for any loss caused by the order.

Courts normally insist on such an undertaking as a condition of granting an injunction. Here, the plaintiffs did not provide one (para. 51), which would have weighed against granting relief had they otherwise met the fair-question threshold.

4.5 Hearsay Evidence at Interlocutory Stage

Hearsay is a statement made outside court, repeated in court to prove the truth of what it asserts. At full trial, hearsay is generally inadmissible except under narrow exceptions.

At interlocutory stage, courts are more flexible: hearsay is often admitted, but:

• The source of the statement should be identified.
• The party relying on it should say they believe it to be true.

In this case, much of the plaintiffs’ hearsay was anonymous, speculative and ventured into expert territory (e.g. medical causation), so the Court attached little weight to it (para. 17).

4.6 Opinion Evidence and Expert Witnesses

Non-expert witnesses may testify about facts they directly observed, but not generally about opinions or specialised inferences. Expert evidence is required when technical or scientific knowledge is needed (e.g. whether a symptom is caused by pesticide exposure).

Here, the plaintiffs sought to give their own views on complex medical and toxicological questions, but as non-experts (and as parties), they were not allowed to do so. Without independent experts, their causation case could not get off the ground at interlocutory stage.

4.7 Plenary Summons and Statement of Claim

A plenary summons initiates civil proceedings in the High Court. It is followed by a statement of claim, which must set out concisely:

• The material facts relied on.
• The legal basis of the claim.
• The remedies sought.

In this case:

• The plenary summons contained unusual language seeking to “prosecute” the defendants for criminal offences.
• No proper statement of claim was served on the defendants; an uploaded document did not comply with the Rules of the Superior Courts and was not treated as a pleading (para. 23).

4.8 Private Prosecutions and Civil Proceedings

While Irish law permits limited forms of private prosecution in the criminal courts, a party cannot “prosecute” an alleged offence by issuing a civil plenary summons in the High Court. Criminal liability and civil liability are distinct.

Here, the plaintiffs’ attempt to prosecute offences under the 1861 Act and the 1991 Act through their civil claim was misconceived, and in any event the relied-on 1861 provisions had been repealed (para. 24).

4.9 Injuries Resolution Board (IRB)

The Injuries Resolution Board (formerly PIAB) is a statutory body that assesses most personal injury claims in Ireland before they can proceed in court. Typically, a plaintiff must first submit their claim to the IRB and obtain an authorisation to sue.

The plaintiffs here referred to difficulties with their GP completing IRB forms (para. 26). Although this might be a procedural issue in a personal injuries case, the defendants did not object on this ground, and the Court did not rely on it as a basis for refusing the injunction (para. 26).

4.10 Statutory Instruments and Presumption of Validity

A Statutory Instrument (SI) is a form of delegated legislation, made by a minister under powers given by an Act of the Oireachtas. S.I. 155/2012 was made to implement EU Directive 2009/128/EC on sustainable pesticide use.

Unless and until successfully challenged, such instruments are presumed valid and lawful. Courts will assume they properly reflect EU and national policy, including safety assessments. Farrell J relies on this presumption when treating authorised pesticide use as lawful and not inherently harmful in the absence of contrary evidence (paras. 41–42).

5. Conclusion

M.A. & Anor v M.C. & Ors [2025] IEHC 618 stands as a clear and carefully reasoned application of interlocutory injunction principles to a contentious environmental health dispute between neighbours.

Its main contributions can be summarised as follows:

• Evidence-driven threshold: Even where plaintiffs sincerely allege threats to life and health from environmental exposures, a court will not grant interim injunctions without at least some cogent medical or expert evidence supporting causation. Belief and correlation are not enough.
• Deference to regulatory frameworks: Where pesticides are lawfully authorised under an EU-informed national regime, courts will assume they are safe when used as regulated, absent evidence to the contrary or a formal challenge to that regime.
• No speculation on central issues: Courts will not speculate on central elements of a claim, such as causation, and will not permit plaintiffs to plug evidential gaps by reference to other decisions or by speculative assertions about neighbours’ health.
• Standards for lay litigants: Lay litigants are held to the same rules on pleadings, evidence and propriety as represented parties. Offensive, speculative and irrelevant material will be struck out and may undermine a party’s credibility.
• Balanced costs approach: While unsuccessful interlocutory applications may result in costs orders, the Court may stay their execution to avoid impeding access to justice in the main proceedings.

For environmental, agricultural and toxic-tort practitioners, the case is a warning that interlocutory relief against lawful land use will not be granted lightly, particularly where it would effectively create new operational constraints (such as pesticide-free buffer zones) absent clear evidence of harm. For lay litigants, it underscores that the High Court is a legal, not a “Private Court”, and that even strongly-felt grievances must be translated into properly pleaded claims supported by admissible, probative evidence.

The underlying proceedings remain on foot, and Farrell J explicitly acknowledges that the plaintiffs might yet prove their case at trial (para. 52) if they can assemble adequate evidence. Nonetheless, this judgment sets an important benchmark: in environmental health injunctions, the “fair question to be tried” test demands at least preliminary expert causation support, particularly when the conduct complained of is otherwise authorised and regulated by law.