Evidence-Based Harmless Error Doctrine in Judicial Review of EU Law Assessments

Introduction

Massey v An Bord Pleanála [No. 4] ([2025] IEHC 309) is the fourth chapter in a long-running High Court challenge by Paddy Massey to the Strategic Infrastructure Development (“SID”) consent granted to Curns Energy Limited by An Bord Pleanála. Over the preceding years Massey contested the Board’s classification of the development as SID, sought certiorari, pursued declarations and triggered a preliminary reference to the Court of Justice of the European Union (“CJEU”) on appropriate assessment (“AA”) questions. In this final instalment he seeks leave to appeal two questions: first, the criteria by which a court may withhold certiorari where AA screening occurred without valid conservation objectives; second, whether section 37A of the Planning & Development Act 2000 is directory or mandatory. Humphreys J. refused leave, reaffirming that the harmless error doctrine—endorsed by the CJEU in Gemeinde Altrip (C-72/12) and described as “well-established” by the Irish Supreme Court—permits courts, on the basis of evidence, to withhold relief where any procedural error makes no practical difference.

Summary of the Judgment

The Court dismissed Massey’s application for leave to appeal. It applied the statutory criteria for leave under sections 50, 50A and 50B of the Planning & Development Act 2000, requiring that any point (i) properly arise, (ii) be a pure question of law, (iii) be of public importance, (iv) be of exceptional importance and (v) that an appeal be in the public interest. Both proposed questions failed on multiple grounds:

The AA question rested on a hypothetical outcome of the CJEU reference, was fact-specific and at best mixed law and fact;
The section 37A point was a minor corporate‐identity technicality, lacking any tangible public interest;
Neither point transcended the record or created genuine doubt in settled law;
Further delay would prejudice the renewable energy project and yield no practical benefit.

On the merits the Court confirmed that where expert or developer evidence establishes no possible impact on European sites, any AA shortcoming is harmless and does not mandate certiorari or remittal.

Analysis

Precedents Cited

Gemeinde Altrip and Others v Land Rheinland-Pfalz (C-72/12, EU:C:2013:712): the CJEU held that procedural irregularities in environmental assessments may be disregarded where a court, based on evidence, is satisfied the error had no effect (“harmless error doctrine”).
Carrownagowan Concern Group v An Bord Pleanála ([2025] IESCDET 9): the Irish Supreme Court described harmless error in judicial review as “well-established”.
Kelly v An Bord Pleanála ([2014] IEHC 400): affirmed that proper AA is a jurisdictional prerequisite but does not render every procedural shortcoming automatically fatal.
Numerous authorities on leave to appeal criteria (e.g. Concerned Residents of Treascon, Nagle View, Monkstown Road Residents), establishing thresholds of public importance, exceptional importance and public interest.

Legal Reasoning

Humphreys J. structured his reasoning around the five statutory requirements for leave to appeal:

Properly arising: Massey’s AA question depended on a hypothetical CJEU answer, went beyond pleadings, and was not determinative in context; the section 37A point had never impaired any right or outcome below.
Pure question of law: The AA issue involved assessment of evidence—a mixed question of fact and law—unsuitable for appeal in planning judicial review. The corporate-identity point was legal but trivial.
Public importance: No genuine doubt existed in the doctrine of harmless error or in the interpretation of section 37A that warranted clarification. Massey manufactured uncertainty.
Exceptional importance: Neither question rose above routine technical contentions. An occasional procedural error must sometimes stand where it causes no harm.
Public interest: Further litigation would delay urgently needed renewable infrastructure, prejudice the developer and serve no wider purpose. Declaratory relief suffices if the CJEU reference alters the legal landscape.

On the AA point, the Court reiterated that where uncontroverted evidence demonstrates no effect on a European site, the harmless error doctrine applies and certiorari is unnecessary. This aligns with Altrip’s sanctioning of post-hoc regularisation and with UK and Irish decisions allowing courts to weigh developer evidence rather than quashing decisions and remitting them wholesale.

Impact

This judgment consolidates a clear, evidence-based approach to procedural errors in EU environmental assessments. Future applicants will face:

High thresholds for leave to appeal on AA defects: must show genuine risk of adverse impact;
Limited scope for abstract or hypothetical challenges to settled AA jurisprudence;
Recognition that demonstration of harmlessness via admissible evidence can bar certiorari;
Emphasis on expedited review in planning cases to protect public interest in infrastructure delivery.

Complex Concepts Simplified

Harmless Error Doctrine: Even if a procedural mistake occurred, a court can refuse to set aside a decision if, based on evidence, the mistake would not have changed the outcome.
Appropriate Assessment (AA): A screening under the Habitats Directive to ensure projects do not adversely affect protected European sites.
Certiorari: A judicial remedy annulling a decision; here, refusal indicates the court finds no need to void the Board’s grant of permission.
Pure Question of Law vs Mixed Question: Pure legal issues may go to appeal; mixed questions requiring factual assessment generally may not.
Leapfrog Appeal: Direct appeal from the High Court to the Supreme Court, bypassing the Court of Appeal, in exceptional circumstances of public importance.

Conclusion

Massey v An Bord Pleanála [No. 4] affirms that the harmless error doctrine, as established by the CJEU and recognised by the Irish Supreme Court, applies robustly in planning judicial review. Where admissible evidence shows no adverse effect on protected sites, procedural deficiencies in AA do not automatically compel certiorari. The decision tightens the criteria for leave to appeal—guarding against speculative or trivial challenges—while balancing the public interest in legal certainty and timely delivery of infrastructure, especially in the context of renewable energy and the climate emergency.