On his return to Belfast, he joined the Irish Republican Army (IRA) and Sinn Féin.
[1] Carolan stood for Sinn Féin in Belfast Shankill at the 1918 Irish general election.
At the 1920 Belfast Corporation election, he was one of five Sinn Féin candidates elected,[3] That year he was one of four anti treaty IRA men arrested in the Gaiety Theatre, Dublin.
[6] An opponent of the Anglo-Irish Treaty, he became Director of Intelligence for the anti-treaty forces in the Irish Civil War, then filled the same post for the surviving IRA.
[7] Frank Kerlin replaced him as Director of Intelligence,[8] but George Gilmore arranged a successful jailbreak in November and Carolan was one of nineteen men sprung in November.