his mother had left an impression on him at the age of seven with her instruction:
"I'm Irish, therefore you're Irish....Don't forget it"
On 25 July 1953, sean took part in an IRA arms raid on the armoury of the Officer Training Corps at Felsted, a public school in Essex. In that raid, the IRA netted over 108 rifles, ten Bren and eight Sten guns, two mortars and dummy mortar bombs. The police seized the van carrying the stolen weapons some hours later due to it being so overloaded that it was going at about 20 mph on the Braintree bypass with a traffic build up in its rear. On 19 August 1953, he was sentenced, along with Cathal Goulding and Manus Canning, to eight years' imprisonment by a court in Bishop's Stortford, Hertfordshire . Seán Mac Stiofáin, Dáithí Ó Conaill and Seamus Twomey together with others established themselves as a "Provisional Army Council", in anticipation of a contentious 1970 Sinn Féin Árd Fheis The "Provisional Army Council" in the coming months would command the loyalty of the IRA national organisation save for a few isolated instances (that of the IRA Company of the Lower Falls road, Belfast under the command of Billy McMillen and other small units in Derry, Newry, Dublin and Wicklow). Mac Stiofáin's men soon came to be known as the Provisional IRA. Sean military strategy was summed up in his own words by "escalate, escalate, escalate" and in 1972, by far the bloodiest year of the conflict, the IRA killed around 100 British soldiers and lost 90 of their own members. On 7 July 1972,Sean Mac Stíofáin led an IRA delegation to a secret meeting with members of the British government, Following the unsuccessful talks, Sean ordered an intensification of the IRA campaign which peaked on 21st July 1972, he was areeted and put in Jailed in the Curragh Prison,for a remark he made on RTE TELEVISION sean immediately embarked on a hunger and thirst strike.,he came of the hunger strike after 15 days. Sean lost his rank upon arrest and he never again regained his influence within the IRA after his release in April 1973 sean was given a job of distribution manager and part-time columnist with the Sinn Féin newspaper An Phoblacht/Republican News in the late 1970s. He resigned from the party in 1982 after a disagreement about strategy at the Ard Fheis.
In the 1980s and 1990s, Mac Stíofáin became active in the Irish language organisation Conradh na Gaeilge. At that organisation’s centenary celebration held in Dublin’s O'Connell Street in 1993, he was a guest of honour on the platform. He remained a member of the standing committee (Coiste Gnó) of Conradh na Gaeilge until his death.