Born in Belfast in an area with a mix of Protestants and Catholics, at an early age, Traynor saw the dead bodies of three Irish Republican Army (IRA) members, all shot in the head. He joined the IRA himself in the 1930s, and served at least two spells in the Crumlin Road Prison, during which he undertook short hunger strikes. By 1938, when the S-Plan was carried out, he was member of its GHQ staff, and for a time, he served as Adjutant-General. Initially known as a bomb maker, alongside Tony D'Arcy, Jack McNeela and Dom Adams, he led agitation for the IRA in the south to lead guerilla raids on the north. When Tomás Ó Dubhghaill suggested raiding the Magazine Store in Phoenix Park, Traynor was his strongest supporter. This was successful, but soon after Traynor was arrested alongside other leading IRA figures while they were meeting at the Meath Hotel in Dublin. Held at Mountjoy Prison and sentenced to three months, Traynor took part in a hunger strike alongside D'Arcy and McNeela. However, after both D'Arcy and McNeela died, it was decided to abandon the protest, Stephen Hayes declaring that they had achieved their aims, although this turned out to be a fiction. In 1942, Traynor was again arrested and was interned in the Curragh; this time, he was kept inside until after the war.