Mona is a leading litigator and Partner in the Byrne Wallace Shields LLP Litigation and Regulation team. Mona heads the firm’s Planning, Environment and Climate practice and she has considerable experience and expertise in advising in planning, environment and climate law matters including the regulatory, legal and policy framework and governing legislation. She regularly advises on public and administrative law mandates and challenges including judicial reviews, statutory appeals, matters of statutory and constitutional interpretation, EU law challenges.  She represents clients in all court jurisdictions including the Court of Justice of the European Union and has particular expertise in the Commercial Court and the Planning & Environment Court. 

In addition to planning, environmental and climate law mandates, Mona advises on all contentious and risk areas including complex litigation, commercial disputes, regulatory mandates, ESG matters, property disputes and contentious procurement. Mona manages a number of large-scale litigation projects as well as portfolios of litigation for public and private sector clients. She is adept at meeting court deadlines and case management directions timetables and is very effective in urgent and complex applications including interim and interlocutory applications. She advises and represents clients on alternative dispute resolution processes including conciliation, mediation and arbitration and is proactive in early dispute resolution.

Key Experience

• Advising An Coimisiún Pleanála in defending judicial review challenges to planning decisions under sections 50, 50A and 50B of the Planning and Development Act 2000, as amended. These cases concerns complex matters of Irish and EU planning, environment and climate law including: (a) the interpretation and application of development plan policies, objectives, land use zoning maps and plan variations, including questions of material contravention; (b) compliance with the EU environmental directive framework: Habitats Directive (92/43/EEC), Birds Directive (2009/147/EC), EIA Directive (85/337/EEC, as amended), SEA Directive (2001/42/EC), Water Framework Directive (2000/60/EC), and the Climate Action and Low Carbon Development Act 2015 (as amended); (c) the application of Ministerial Guidelines under section 28 of the 2000 Act; (d) Appropriate Assessment compliance, including screening for likely significant effect and the standard of no adverse effect on integrity; (e) Environmental Impact Assessment, including screening and preliminary examination; and (f) fair procedures, duty to give reasons, public participation, and irrationality in decision-making.  Mona led the team in proceedings before the Supreme Court in the seminal Coolglass case, a case concerning the meaning and interpretation of section 15 of the Climate Action and Low Carbon Development Act 2015 (as amended). The Supreme Court held that the climate obligation in section 15(1) applies directly to all public bodies in the exercise of their statutory functions.
• Advising a public body in judicial review proceedings relating to compulsory purchase applications made under the Housing Act, 1966 (as amended).  Mona’s work includes advising on arguments relating to the application of the incorrect legal test, improper considerations as to the suitability of land for acquisition, matters of fair procedures and the proper conduct of oral hearings, contested hearings in relation to the application of cost protection to CPO judicial reviews.
• Key advisor to the Health Service Executive in a number of significant mandates including Covid-19 projects and the integration between the private hospitals and the public healthcare system due to the healthcare capacity constraints as well as the response to the cyber-attack on the HSE’s systems advising on legal strategy and risk, managing review of data and the response and strategy to claims.  Also managing a portfolio of contentious matters including Commercial Court and Court of Appeal cases and regulatory advices.
• Acting for Inland Fisheries Ireland in Commercial Court judicial review proceedings challenging the decision to grant an aquaculture licence;
• Acting for the NTPF in Judicial Review proceedings issued by a nursing home provider relating to the fair deal price offered to the nursing home.
• Acting for HSE, Fáilte Ireland, Beaumont Hospital & private clients in numerous High Court judicial review procurement cases on the award of public contracts under EU Remedies Regulations/procurement law.
• Lead Partner advising the Ballymore Group, in high profile and complex High Court litigation against CRH plc and its subsidiary Roadstone Ltd, as well as other construction product suppliers concerning the supply of defective products.
• Advising Horse Racing Ireland on its statutory functions and powers and legislative provisions governing its operations.  Advising on risks in strategic decisions, governance and its public body role and mitigating against legal challenge/judicial review.
• Acting for the Financial Services and Pensions Ombudsman in defence of High Court statutory appeals of its decisions and in defence of judicial review proceedings.
• Acting for The Courts Service of Ireland in a number of contentious matters including seminal Freedom of Environmental Information case with a preliminary reference to the Court of Justice of the European Union.
• Managing disputes for retailer and property developer clients including urgent injunction applications, property disputes and breach of contract claims.
• Advising public body and private clients on their ESG obligations, governing regulations and EU Directives, reporting obligations, green procurement requirements and the management of ESG risks. Mona also chairs the firm’s ESG cross-departmental practice group.
• Advising Board of Directors of private companies in governance matters, potential misappropriation of funds, alleged fraudulent activity of employees. Advices on investigation processes and inquiries, drafting terms of reference, retaining appropriate experts, engagements with regulatory authorities, advising on findings and responses.
• Advising on a major tracker based investigation for an Irish bank. Appointed as Central Bank of Ireland Independent Assessor to assess and report on adequacy of the investigation.
• Advising public body clients on the Code of Practice for the Governance of State Bodies and the State Litigation Principles. 

Awards

• Awarded the prestigious Norman Bale Irish Tax Review Article of the Year Award, 2022 for her article with her ByrneWallace Tax Partner colleague, Lee Squires, “Arderin Distillery: Legitimate Expectation and Judicial Review of Revenue.”
• Mona has also previously won the Litigation Case of the Year at the Irish Law Awards for her work on the Court of Justice of the European Union Waterford Crystal case.

Qualifications 

• Masters of Law (LL.M) Trinity College Dublin, 2000
• BA, Law and European Studies, University of Limerick, 1999  

Memberships 

• Dublin Solicitors Bar Association
• Law Society of Ireland
• Association of Regulatory and Disciplinary Lawyers
• Commercial Litigation Association of Ireland

Recommendations

• ‘Mona Costello particularities stand out. They have provided an excellent service to us throughout the management of follow up actions arising from a cyber attack. They have been reliable, consistent and very professional in all their dealings.’ - Legal 500, 2025
• 'Mona Costelloe and Sean O'Donnell are excellent litigators, who are working on novel areas of data protection law.' - Legal 500, 2024
• 'I have found Mona Costelloe, Grainne Murphy and Robert McDermott diligent and careful in their advices which will hopefully leave us in good stead when the issues come to be resolved.' - Legal 500, 2023
• Mona Costelloe is 'a specialist in Commercial Court proceedings' - Legal 500, 2023
• 'Mona Costelloe is ‘highly professional and a delight to deal with’' - Legal 500, 2022
• 'Mona Costelloe is 'particularly impressive'' - Legal 500, 2022
• Mona Costelloe 'is known for her work on property ligitation' - Legal 500, 2021