By the late 1960s, the teenaged O'Callaghan had ceased practising the Catholic faith, adopted atheism and had become interested in the theories of Marxist revolutionary politics, which found an outlet of practical expression in the sectarian social unrest in Northern Ireland at that time, centred on the activities of the Northern Ireland Civil Rights Association. In 1969, an outbreak of communal violence broke out in Northern Ireland and believing that British imperialism was responsible, O'Callaghan joined the newly founded Provisional IRA at the age of 17.