MacGrath attended University College Dublin from where he received BCL and LLM degrees.
[4][5] In addition to his legal practice, he was the chairperson of the Broadcasting Complaints Commission between 2005 and 2009,[2] and was the chair of the Mining Board from 2013.
[14] In the midst of a judicial review hearing in 2019, a lay litigant unsuccessfully attempted to effect a citizen's arrest against him.
[15] In 2019, he decided against the Friends of the Irish Environment which sought to challenge governmental policy on climate change and reducing carbon emissions.
[17] He was elevated as a judge of the Court of Appeal in July 2024, and was appointed by President Michael D. Higgins.