Known as "Thumper" owing to his short temper and habit of banging his fist on tables, he received little education and was a bookmaker's (bookie's) 'runner'.

He began his involvement with the militant youth organisation, Fianna Éireann, in 1936 before joining the Irish Republican Army in the 1937.

[2] He opposed the left-wing shift of Cathal Goulding in the 1960s, and in 1968, helped set up the breakaway Andersonstown Republican Club (later the Roddy McCorley Society).

[5] By 1972, he was Officer Commanding of the Provisional IRA's Belfast Brigade when it launched its bombing campaign of the city, including Bloody Friday when nine people were killed.

In an interview with French television on 11 July 1977, he declared that although the IRA had waged a campaign for seven years at that point, it could fight on for another 70 against the British state in Northern Ireland and in England.

[11] In December 1977, he was captured in Sandycove, Dublin by the Garda Síochána, who had been tipped off by Belgian police about a concealed arms shipment, to be delivered to a bogus company with an address in the area.

The Gardaí later found documents in his possession outlining proposals for the structural reorganisation of the IRA according to the clandestine cell system.