Introduction

The judgment in Doyle & Ors v An Bord Pleanala & Ors ([2025] IEHC 158), delivered by Humphreys J. on 21 March 2025 in the High Court of Ireland, addresses the intersection of environmental protection, planning law, and the doctrine of harmless error. Central to the case is an allegation that the Board conducting the Environmental Impact Assessment (EIA) for a complex development project had made a material error concerning the treatment of the bat fauna present on the site, particularly relating to a single confirmed Leisler’s bat roost in Building 6C.

The dispute arose from two planning applications – one for a data centre development and another for a 110kV GIS substation – both approved by An Bord Pleanala. The applicants challenged these decisions on several grounds, with a focal point being the alleged mischaracterization in the Inspector’s report regarding the loss of a bat roost. The inspector’s report stated that no bat roosts were confirmed, despite evidence that a roost of a single Leisler’s bat had been identified in Building 6C during surveys, raising questions about the adequacy of the EIA and the Board’s subsequent certification of no significant adverse effects on bat biodiversity.

This commentary explores the background, key issues, and legal reasoning underlying the Board’s decision to rely on robust mitigation measures and the harmless error doctrine, despite the factual inaccuracy noted in the ecological surveys.

Summary of the Judgment

Humphreys J. delivered a comprehensive judgment that ultimately dismissed the applicants’ challenge for the module addressing the alleged error concerning bat roost loss. The Court held that the error in the Inspector’s report – the oversight concerning the confirmed presence of a single Leisler’s bat roost in Building 6C – was “harmless” in the overall context of the EIA and the Board’s decision.

Key points from the judgment include:

• The inspector’s acknowledgement that the revised EIAR had recorded the presence of a solitary bat roost was considered immaterial because the overall ecological assessment had factored in the species’ high mobility and potential for roost switching.
• The comprehensive mitigation measures, including conditions requiring a pre-construction bat survey, the implementation of compensatory planting, and the need for a derogation licence if roost destruction occurred, ensured that any environmental impact was effectively managed.
• The Court reiterated that under both domestic and EU law, not every factual or procedural error warrants the quashing of a decision, provided that a reasonable certainty exists that the overall outcome would remain unaffected – the essence of the harmless error doctrine.
• Modularisation of the judicial review process meant that the current module was examined in isolation, assuming the validity of the decision on all other grounds, thereby limiting the scope of the challenge to sub-grounds 91–93.

Analysis

Precedents Cited

A number of important precedents were cited in the judgment. The Court referenced key cases such as:

• Carrownagowan Concern Group v. An Bord Pleanála (No. 3) – This case was instrumental in discussing the role of harmless error and the circumstances under which a factual inaccuracy may not justify quashing a planning decision.
• Altrip (ECLI:EU:C:2013:422) – The decision in Altrip played a pivotal role by laying out the principles regarding procedural defects, the reliance on evidence provided by interested parties, and the need to exclude reasonable doubt as to whether an error would have affected the outcome.
• Eco Advocacy v An Bord Pleanála [2025] IEHC 15 – This decision underscored that environmental mitigation measures can effectively neutralize the impact of minor errors and affirmed that judicial discretion should not be exercised lightly in quashing decisions.

Collectively, these precedents shaped the Court’s reasoning by providing a framework in which the harmless error doctrine was applied. The judgment noted that while the error concerning the bat roost was factual in nature, prior jurisprudence demonstrated that such defects are not automatically fatal if they do not influence the overall decision‐making process.

Legal Reasoning

The Court’s legal reasoning focused on the doctrine of harmless error. Central to the argument was the principle that:

“A decision should not be quashed for error (including in the application of EU law) if the error was harmless and did not materially affect the result.”

In applying this principle, the Court considered several factors:

• The scientific evidence established that bats, including Leisler’s bat, exhibit highly mobile behavior with frequent roost switching. This ecological reality meant that the detection of a single roost in Building 6C was not, in itself, dispositive of a significant environmental impact.
• The EIA and the consolidated mitigation measures were constructed based on a “roost resource approach” that covered both confirmed and potential roost sites. The comprehensive measures contemplated the possibility that bat roosts may change or be reoccupied during the construction period.
• The Board had conditioned its grant of permission on stringent requirements, including additional surveys, a pre-construction check, and the potential need for a derogation licence. These steps ensured that the environmental risks, particularly those affecting bat fauna, were adequately mitigated.
• The modular approach to judicial review enabled the Court to isolate the harmless error issue from other grounds of challenge. This procedural separation ensured that the error in question was analyzed on its own merit, without conflating it with broader issues raised by the applicants.

Therefore, the court concluded that the factual error did not upset the fundamental conclusion that the development would not have significant adverse environmental effects; rather, it was subsumed under the umbrella of acceptable administrative error given the robust mitigation framework.

Impact on Future Cases and Relevant Areas of Law

This judgment is significant because it reinforces the strength of the harmless error doctrine in both domestic and EU contexts of planning and environmental law. The judgment sets a precedent that:

• Not every factual error in environmental impact assessments necessitates overturning a planning decision, especially in cases where comprehensive mitigation measures are in place.
• The approach to assessing environmental impacts must take into account the dynamic and inherently unpredictable nature of ecology – the “roost resource approach” provides flexibility in planning decisions.
• Future challenges to planning permissions on technical or factual grounds will likely be evaluated in light of whether such errors could have reasonably altered the outcome. This positions decision-makers with a broader discretion to rely on expert evidence and pre-existing methodologies.

Complex Concepts Simplified

Several complex legal and scientific concepts underpinned the judgment. For clarity:

• Harmless Error Doctrine: This legal principle holds that if a mistake in a decision-making process is not influential in altering the outcome, it should not be used as grounds to overturn that decision. Essentially, a minor error that does not lead to an unjust result is considered “harmless.”
• Roost Resource Approach: Rather than relying solely on the existence of confirmed roosts at the time of survey, this method considers all potential roost sites – both confirmed and unconfirmed – and incorporates the natural behavior of bats, including their frequent switching between roosts. This approach aims at ensuring that remediation measures are robust in a dynamic ecological environment.
• Modularisation in Judicial Review: This procedure involves addressing different legal challenges in separate “modules” so that issues that have been conceded or are deemed harmless in one module do not adversely affect the evaluation of other grounds in subsequent modules.

Conclusion

In conclusion, the judgment in Doyle & Ors v An Bord Pleanala & Ors ([2025] IEHC 158) represents a significant development in the application of the harmless error doctrine within the context of environmental and planning law in Ireland. The Court’s rigorous analysis demonstrates that—even where a factual error regarding the presence of a bat roost is identified—if the comprehensive mitigation measures and robust assessment methodologies are in place, the overall decision need not be quashed.

The judgment not only clarifies the proper approach to balancing environmental protection with practical realities of development but also offers valuable guidance for future cases. By reinforcing that minor procedural or factual errors, when deemed harmless, should not disrupt the legitimacy of a decision, the ruling promotes a reasoned, evidence-based evaluation and underscores the importance of expert methodologies in ecological assessments.

Ultimately, the Court’s decision is a reminder that while environmental protection remains paramount, planning authorities must also be afforded the necessary discretion to manage complex scientific uncertainties and administrative details without being unduly hampered by minor errors.