[3] While attending University College Dublin, he won the 1978 Irish Times Debate with Conor Gearty for the UCD Law Society.

[4] He graduated from Virginia in 1983, where he wrote a research paper comparing equality under the US and Irish constitutions, supervised by A.E.

[13] He acted for the State in Zappone v. Revenue Commissioners, Roche v Roche and Miss D.[9][2] In Michael Ring's challenge to a ban on a dual mandate he acted for the State and represented Micheál Martin in an action taken by Kathy Sinnott challenging the results of the 2002 general election in Cork South-Central.

[18][19] He represented the Beef Industry Development Society Ltd in a 2008 case in the ECJ which clarified the meaning of an agreement under Article 101 of the Treaty on the Functioning of the European Union.

[20] In 2007 he and Paul Anthony McDermott acted for Elin Nordegren in libel proceedings against The Dubliner.

[27] There have been several distinctive and innovative features of his Supreme Court judgments, including writing joint opinions and opting to delay a declaration of unconstitutionality, instead of no declaration at all, to enable the government to take action before a judgment takes effect.

[11] Ruadhán Mac Cormaic of The Irish Times says O'Donnell has a reputation for elegant writing and having a "socially liberal" approach.

[11] He was reported to have been one of three judges shortlisted to be the 12th Chief Justice of Ireland in July 2017; however, Frank Clarke was chosen.