A friend and confidant of Sinn Fein president Gerry Adams, Cahill played a key role in getting backing for the peace process in later years.
Mr Adams described him as “an unapologetic physical force republican who fought when he felt that was the only option, but he also stood significantly for peace”.
In the early 1970s, as IRA chief-of-staff, Cahill was sprung from Mountjoy Prison when a hijacked helicopter landed in the exercise yard in broad daylight.
The veteran republican was sentenced to hang with three colleagues in 1942 after an IRA operation in west Belfast resulted in the death of Catholic police constable Patrick Murphy. Tom Williams, leader of the IRA unit, took full responsibility and was executed.
Cahill suffered from asbestosis, acquired while working in Belfast’s Harland & Wolff shipyards in the 1950s.
After the IRA split in 1969/70 he became first chief-of-staff of the Provisionals. Sixteen years later he backed Mr Adams and Martin McGuinness when Sinn Féin abandoned the traditional Dáil abstentionist policy.
He is survived by wife Annie and family.