Judicial Review: Cross-Examination of Expert Affidavits Requires a Genuine, Material Dispute of Opinion (Not Legal Argument)

1) Introduction

In Friends Of The Irish Environment CLG and others v Environmental Protection Agency [2026] IEHC 415, the High Court (Farrell J) determined an interlocutory application for liberty to cross-examine an expert deponent (Prof. Fawell) who had sworn affidavits on behalf of a notice party (Uisce Éireann) in judicial review proceedings. The applicants—an environmental NGO and two individual applicants—challenged matters arising from drinking water compliance and alleged health risks associated with manganese exceedances in the supply served by the Lee Road water treatment plant.

The immediate question was not the substantive legality of the EPA’s position, but whether cross-examination of a notice party’s expert was required to resolve conflicts in affidavit evidence so that the court could decide a material issue in the judicial review.

Key issues

  • When is cross-examination permissible in judicial review, particularly where the contested affidavits are expert opinions?
  • Do asserted disagreements about the precautionary principle, the status of WHO Guidelines, and the interpretation of EU drinking water rules amount to “genuine disputes” suitable for cross-examination?
  • How does the court distinguish between (a) conflicts of evidence requiring oral testing and (b) matters for legal submission?

2) Summary of the Judgment

The court refused the application for cross-examination. While noting the application could and should have been brought earlier, Farrell J held the delay did not of itself disentitle the applicants. The decisive point was that the applicants failed to show that cross-examination was necessary to resolve a genuine dispute in expert opinion that had to be determined in order to decide a material issue in the judicial review.

Farrell J found that the seven identified “issues” largely concerned:

  • inferences drawn from public advisories (not expert opinion),
  • argument about the legal status of WHO Guidelines,
  • interpretation and application of the Drinking Water Directive and the European Union (Drinking Water) Regulations 2023, and
  • the meaning and operation of the precautionary principle (principally a matter of law).

The court indicated a limited reserve power: if, after determining the legal issues, cross-examination later proved essential to decide the proceedings, the court would revisit the question—without inviting a further application.

3) Analysis

A) Precedents Cited

i) RAS Medical v. RCSI [2019] IESC 4; [2019] 1 I.R. 63; [2019] 2 I.L.R.M. 273

Farrell J treated RAS Medical as central authority for the proposition that cross-examination on affidavit in judicial review is exceptional. Although the judgment here does not quote at length from RAS Medical, it underpins the disciplined approach: judicial review is ordinarily determined on affidavit and legal submissions, and oral evidence is permitted only where truly necessary to resolve a dispute that the court cannot fairly decide on paper.

ii) Hegarty v. Commissioner of An Garda Siochana [2021] IECA 328

Farrell J relied on Noonan J’s structured limits on cross-examination in judicial review:

  • Genuine dispute requirement: it is insufficient to simply assert non-acceptance of the respondent’s factual account without putting forward an alternative factual case.
  • Materiality/necessity requirement: even where there are opposing factual versions, cross-examination is generally only permitted where resolving the conflict is essential to determining the legal issues.
  • Anti-fishing principle (para. 43 of Hegarty): cross-examination cannot be used to elicit new facts to build new grounds of challenge; that would “subvert the judicial review process”.

These principles directly shaped the court’s assessment of whether the asserted expert disagreements were (a) genuine disputes and (b) necessary to decide an outcome-influencing issue.

iii) Salmon Watch Ireland CLG v. Aquaculture Licences Appeals Board [2023] IEHC 129

Farrell J adopted Holland J’s formulation (para. 5) that where a conflict of affidavit evidence requires the court to prefer one witness over another, it is unfair to do so without giving the other witness an opportunity, by cross-examination, to respond and explain why their evidence should be preferred. Importantly, Farrell J also relied on Salmon Watch for the expert-specific proposition that cross-examination may be necessary where expert opinions (not just underlying facts) are genuinely in dispute and resolution is material to the case.

iv) Ethical Farming Ireland v. Minister for Agriculture & Ors [2026] IEHC 42

The notice party relied heavily on Cahill J’s summary that cross-examination is permissible where there is a genuine conflict of fact or a credibility/reliability challenge that must be resolved for determination of an issue; and that bare denials and challenges to interpretations, inferences, opinions, or submissions on affidavit are not a basis for cross-examination.

Farrell J clarified the limits of reliance on that dictum: Ethical Farming Ireland had expressly excluded the question of cross-examination of experts, and was premised on conflicts of fact. Farrell J nonetheless accepted the broader point that courts do not rely on “submissions” contained in affidavits (legal argument, inferences, and interpretive gloss), and therefore cross-examination is not a vehicle to contest such matters.

B) Legal Reasoning

i) The governing test: rarity, genuineness, and necessity

The judgment synthesises the authorities into a practical test:

  • Cross-examination in judicial review is rare and “rarely appropriate”.
  • It may be appropriate (even essential) where there is a genuine dispute that must be resolved to decide a material issue.
  • It is not permitted to (a) fill evidential gaps, (b) broaden inquiry into a roving exploration, or (c) elicit new material to formulate additional grounds.

ii) Expert disputes: opinion can qualify, but must be real and outcome-relevant

Farrell J expressly accepted that disputes between expert opinions can justify cross-examination, not only disputes about primary facts or data. However, the judge insisted on two constraints:

  • The dispute must be truly one of expert opinion (as opposed to legal interpretation or argumentative inference).
  • Resolving it must be necessary to determine an issue “capable of influencing the outcome”.

iii) Application to the seven proposed issues: legal questions dressed as expert conflict

Farrell J’s issue-by-issue approach illustrates a key discipline: separating (a) admissible expert assistance from (b) matters reserved to the court as questions of law.

  • Issue 1 (public advisory inference): The court held that inferences drawn from Uisce Éireann’s advice (that water is safe “once it runs clear”) were not expert opinion and did not create a genuine expert conflict. The judge treated the impugned statement as supposition/extrapolation rather than a disputable expert conclusion.
  • Issues 2–5 (WHO Guidelines; Directive interpretation; precautionary principle): The court repeatedly characterised these as matters of legal argument—e.g., whether WHO Guidelines are standards, how EU limits are to be interpreted/applied, and what the precautionary principle requires under the governing legal regime.
  • Issues 6–7 (risk thresholds; uncertainty; “precaution” framing): While these came closest to scientific controversy, the court found no genuine expert dispute requiring oral testing. There was no dispute that manganese ingestion at excessive levels can pose risks, particularly for infants and children, or that epidemiological studies exist. The real contention was the legal implication of uncertainty and whether precaution requires proof of safety below a figure (e.g., 80 μg/l). Farrell J treated that as principally legal evaluation rather than a conflict resolvable by cross-examination.

iv) A restrained procedural safeguard: possible revisiting

Although refusing cross-examination, Farrell J acknowledged that legal conclusions can affect whether factual/expert disputes become outcome-determinative. The court therefore left open the possibility of revisiting the need for cross-examination after resolving the legal issues—while cautioning that this was not an invitation to renew the motion.

C) Impact

The decision reinforces a firm procedural boundary in Irish judicial review:

  • Expert affidavits do not convert judicial review into a trial: even where public health and scientific assessment are prominent, cross-examination is not granted unless the dispute is genuine, evidential, and material to outcome.
  • Legal questions cannot be smuggled into cross-examination: arguments about the precautionary principle, the legal force of guidelines, and directive interpretation are for submissions, not oral interrogation of experts.
  • Case-management realism: the court’s discussion of timing and availability shows willingness to accommodate cross-examination where justified, but refusal here signals that convenience alone cannot justify it.

For future environmental and public health judicial reviews, the judgment encourages parties to:

  • identify with precision the material issue that cannot be decided without choosing between competing expert opinions;
  • demonstrate a real conflict in opinion (methodology, assumptions, application to facts), not merely disagreement with legal consequences; and
  • avoid using cross-examination as a tool to expand grounds or seek clarificatory elaboration that should be contained in affidavit exchange and written submissions.

4) Complex Concepts Simplified

  • Judicial review: a court review of the legality of a decision/action (process, powers, reasonableness, legal standards), usually based on affidavit evidence rather than oral testimony.
  • Affidavit evidence: written sworn evidence. In judicial review, it is the normal evidential format.
  • Cross-examination: oral questioning of a witness in court to test reliability, credibility, or the soundness of evidence. In judicial review it is exceptional.
  • Genuine dispute of fact/opinion: a real conflict between competing accounts or expert conclusions, supported by alternative evidence—not mere non-acceptance.
  • Materiality / necessity: the dispute must matter to the outcome and must be one the court cannot fairly resolve without oral testing.
  • Precautionary principle: a legal principle (particularly in EU environmental/health contexts) addressing how decision-makers should act under scientific uncertainty. This judgment treats its content and application primarily as questions of law for the court.
  • WHO Guidelines vs legal standards: guidelines are scientific guidance documents; legal standards are binding limits set by legislation/regulation. Whether guidelines have legal force in a given regime is a legal question.

5) Conclusion

[2026] IEHC 415 is a careful restatement and application of Irish principles governing cross-examination in judicial review, with particular relevance to expert evidence in environmental and public health disputes. Farrell J confirms that:

  • cross-examination remains exceptional;
  • it requires a genuine and material conflict of evidence—expert opinion included;
  • it cannot be used to litigate legal interpretation (including the precautionary principle) through witnesses; and
  • courts will police the boundary between evidence and argument to preserve the character of judicial review.