[1] She attended University College Dublin and the King's Inns, graduating from UCD with a BCL degree in 1988.
[8] She was prosecution counsel in cases against Seán FitzPatrick relating to Anglo Irish Bank,[9] Sharon Collins in hiring a hitman to kill her partner,[10] Mark Nash regarding the Grangegorman killings,[11] and Linda and Charlotte Mulhall,[12] She was also involved in prosecutions in the Special Criminal Court,[13] including a trial related to the Omagh bombing.
[2] She is a former director of the Irish Council for Civil Liberties and chairperson of the Private Securities Services Appeal Board.
[2] She has co-authored books on the law of evidence in Ireland and the Special Criminal Court.
The judge refused reliefs sought by O'Brien against the politicians, finding that there could be no judicial interference in regulating parliamentary privilege in the Irish legislature.
[29] She has served as an ad hoc judge for Ireland in the European Court of Human Rights.