Sheehan was born in Belfast, Northern Ireland. He attended St Malachy's College and gained several O-levels before dropping out of full-time education.[3] In his youth he was a member of Fianna, the IRA youth wing.[4]
In 1978—aged 19—Sheehan took part in the bombing of cash-and-carry.[5] He was sentenced to 15 years.[6]
Sheehan was the 17th republican inmate at the Maze Prison to join the 1981 hunger strikes, which was aimed at gaining political status for Provisional IRA and Irish National Liberation Army prisoners. Sheehan began fasting on 10 August – after nine prisoners had already died – and ended when the hunger strike was officially called off on 3 October. He survived 55 days without food.
Sheehan was released in 1987. In 1989, he was convicted and sentenced to 24 years for attempting to booby trap a Belfast security checkpoint with a bomb.[7] Sheehan spent his first two years at Crumlin Road Gaol, before being moved to H-Block, sharing a cell with Danny Morrison.[8] He was released under the terms of the 1998 Belfast Agreement.
He picked up his education again on his release and in 1992 began to study Social Sciences and Philosophy with The Open University. He graduated with a first class honours degree in 1998.[9]
Sheehan has provoked anger and controversy by describing the Troubles as "probably quite civilised" and saying the IRA "could have left a 1,000lb car bomb on the Shankill" if it wanted to kill Protestants.[10]