McBirney studied law at Trinity College Dublin and became a barrister in Belfast, before being called to the bar at Gray's Inn. In his spare time, he became active in the Northern Ireland Labour Party (NILP), and served on its executive from 1964-67. In 1966-67, he was also party Chairman. He combined these roles as the first Chairman of the Northern Ireland Society of Labour Lawyers.
McBirney stood for the NILP in Belfast Willowfield at the 1965 Northern Ireland general election, taking 35.1% of the vote. He was next a candidate in East Belfast at the 1966 general election, and took 45.3% of the vote in one of the party's best ever results, although he was not elected. His final contest was Belfast Pottinger at the 1969 Northern Ireland general election, where he took 48.6% of the vote and was fewer than two hundred votes short of being elected.
In the 1970s, McBirney focused on his legal career. He was made a Queen's Counsel, was appointed a resident magistrate, and served as Senior Crown Prosecutor for Belfast. He worked as a lawyer for the defence on civil rights cases, including one involving the Derry Citizens' Action Committee, in which he defended, among others, Eamonn McCann.
On the morning of 16 September 1974, McBirney was shot dead at his house on Belmont Road, East Belfast. He was killed in the attack, on the same day that another judge, Rory Conaghan, was murdered. Both murders have been attributed to the Irish Republican Army. On hearing of McBirney's death, his sister-in-law, Frances Cooke, suffered a fatal heart attack.
McBirney was a Protestant who had married a Roman Catholic. The couple had one child. He was friendly with politician Paddy Devlin, with playwright Sam Thompson and poet Louis MacNeice. He personally wrote documentaries and plays for the BBC, including one on Daniel O'Connell. McBirney appeared as a contestant on the Round Britain Quiz.
- The Irish Law Times and Solicitors' Journal, vol. 81, p. 118
- Aaron Edwards, A history of the Northern Ireland Labour Party, p.157
- David McKittrick, Lost Lives, pp.474-475
- "Northern Ireland Parliamentary Election Results: Boroughs - Belfast", United Kingdom Election Results
- "East Belfast 1950-1970", Northern Ireland Elections.
- Fionnbarra Ó Dochartaigh, Ulster's White Negroes, p. 61
- Northern Ireland: Terrorist Activities", Hansard; accessed 7 October 2015.
- Kevin Mullan, "Murder of city-born Judge raised with Kissinger", Londonderry Sentinel; accessed 7 October 2015.
- Malcolm Sutton, "An Index of Deaths from Conflict in Ireland: M", CAIN Web Service
- Susan McKay, Bear in Mind These Dead, p. 206
- Connal Parr, "Managing His Aspirations: The Labour and Republican Politics of Paddy Devlin", Irish Political Studies, 27:1
|Party political offices|
| Chairman of the Northern Ireland Labour Party