He was called to the Bar in July 1984 and became a Senior Counsel in 1997. He appeared domestically in cases in the High Court and the Supreme Court of Ireland and internationally at the European Court of Human Rights and the European Court of Justice. Hogan is noted in particular for his experience in constitutional law. He acted for the Attorney General of Ireland in references made by President Mary Robinson under Article 26 of the Constitution of Ireland to the Supreme Court regarding the Information Bill 1995 and the Employment Equality Bill of 1997. He appeared again for the Attorney General in another reference made by President Mary McAleese regarding the Health Bill 2004. He was a law lecturer and fellow in Trinity College Dublin from 1982 to 2007. He lectured on constitutional law, competition law and the law of tort. He is regarded as "one of the foremost constitutional and administrative lawyers in Ireland". He is the co-author of Administrative Law in Ireland and JM Kelly: The Irish Constitution, the core Irish legal texts in Irish administrative and constitutional law respectively. He has also written a text on political violence and a book where he chronicled the origins of the Constitution of Ireland. During his career as a barrister he was involved in cases involving employment law, habeas corpus, immigration law, judicial review, company law, and commercial law. He appeared for Katherine Zappone and Ann Louise Gilligan in Zappone v. Revenue Commissioners in the High Court and Miss D in her case related to the rights to travel abroad for an abortion. He represented the State in the High Court and the Supreme Court in litigation that emerged following a court finding that an offence of unlawful carnal knowledge was unconstitutional. In 2008, he acted for Colm Murphy and Seamus Daly in the Supreme Court who were contesting an action taken by families of victims of the Omagh bombing when they were refused access to books of evidence. Hogan was involved in several tribunals and Oireachtas committee investigations, appearing either in the actual proceedings or in related court proceedings. He represented Desmond O'Malley at the Beef Tribunal in 1992, Dermot Desmond at the Moriarty Tribunal in 2004, and Jim Higgins and Brendan Howlin in actions related to the Morris Tribunal. He acted for the Committee on Members’ Interests of Seanad Éireann in action taken by Ivor Callely. He was the first barrister to appear in an Irish court without a wig, following the enactment of the Courts and Court Officers Act 1995. Throughout his career, he has been a member of committees and boards in areas requiring legal expertise. He chaired the Department of Justice and Equality's Balance in Criminal Law Review Group, and was a member of three other review groups: the Constitution Review Group, the Competition and Mergers Review Group and the Offences Against the State Acts Review Group. He was also a member of the Competition Authority's Advisory Panel and the Committee on Court Practice and Procedure.
He subsequently became a Judge of the Court of Appeal upon its establishment in October 2014.
Advocate General
In May 2018, he was nominated by the Government of Ireland for appointment as the Advocate General to the European Court of Justice. His term began in October 2018 and will expire in October 2024. In one of his first opinions, on a reference from the French Conseil d'État, he found that Regulation No 1169/2011 of the European Parliament and of the Council of 25 October 2011 requires that products originating from Israeli-occupied territories should indicate if these products come from such a territory. His opinion was followed by the Court of Justice.
