Our Team

Mona Costelloe
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Mona Costelloe is a Partner in the ByrneWallace LLP Litigation & Dispute Resolution team. She advises a wide range of national and international clients on complex litigation issues, including financial services litigation, administrative law and contentious regulatory matters, contentious procurement, judicial reviews, EU law challenges, property disputes and contentious construction disputes, product liability and sale of goods claims.

Mona represents clients in all Court jurisdictions with particular expertise in the Commercial Court. She advises and represents clients in the Alternative Dispute Resolution process, including conciliation, mediation, and arbitration, and is proactive in attempting to achieve early dispute resolution.

Mona has presented internal and external seminars and has previously lectured law modules in Dublin City University and Dublin Institute of Technology.

Recent Experience

  • Advising on a major tracker based investigation mandated by the Central Bank of Ireland. Appointed as Independent Assessor to assess and report on adequacy of the investigation.   Reviewing and assessing legal and regulatory reviews of the banks’ loan book, assessing appropriateness and adequacy of redress and compensation proposals, reporting and engaging with the Central Bank of Ireland.
  • Representing the Irish League of Credit Unions and various Credit Unions in High Court judicial review proceedings of the decisions of the Central Bank as they related to various regulatory directions issued under the Credit Union Act 1997 (as amended). Advising on all aspects of the litigation process and the judicial review procedure.
  • Representing Credit Unions in Statutory Appeals before the Irish Financial Services Appeals Tribunal. 
  • Advising unsuccessful tenderers in public contracts and public frameworks, interrogation of tender process, identification of breaches of EU law and procurement rules drafting Regulation 8 pre-action correspondence, advices on merits and acting in High Court challenge and the Order 84A of the Rules of the Superior Courts public procurement judicial review process.   
  • Acting for public bodies in defence of High Court public procurement judicial review actions pursuant to the EU Remedies Regulations, relating to the award of public contracts. Advices relate to litigation process, the special procedure in public procurement judicial review cases, the provisions of the Remedies Regulations and applications for interlocutory relief and public procurement High Court judicial review actions. 
  • Representing a national retail client in a Supreme Court appeal and Court of Justice of the European Union (CJEU) preliminary reference proceedings concerning infringement of Community unregistered design rights.  This was the first case on Community unregistered design rights to come before the Courts of an EU Member State. The Supreme Court referred a number of questions of EU law to the CJEU in a preliminary reference under Article 267 of the Treaty on the Functioning of the EU. Two Supreme Court Judgments were delivered in the proceedings and a decision of the CJEU.
  • Acting for Areva Group in High Court competition proceedings concerning a follow-on damages claim by ESB for losses suffered as a result of competition infringement (cartel) in the gas insulated switchgear (GIS) market. Issues included, global cartel in GIS market, complex competition infringement decisions of the European Commission, the General Court and the CJEU; advices on provisions of Irish and EU Competition law and Irish Court process, indemnity and contribution and damages issues.
  • Managing a portfolio of High Court/Commercial Court breach of contract and product liability cases for a number if private clients for damage to houses due to swelling of pyritic infill including case management of proceedings, briefing Counsel, engaging team of international experts, drafting pleadings and witness statements, supervising large scale discovery exercises, advising on alternative dispute resolution including successful mediation of lengthy Commercial Court proceedings.
  • Representing a group of pension scheme members in Commercial Court and CJEU proceedings on the alleged failure of the Minister for Social and Family Affairs to adequately transpose the provisions of European insolvency directives concerning the protection of employees and their pension entitlements in the event of a double insolvency. Successful judgment on all legal matters obtained from the CJEU and ultimately implemented in Ireland. The effect on the Irish State of having to adequately transpose the directive estimated to be well in excess of €100M.


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


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


"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 ‘highly professional and a delight to deal with’ - Legal 500, 2022

Mona Costelloe is 'particularly impressive' - Legal 500, 2022