He has been active in republican circles since the 1970s. He was Director of Elections in the Anti H-Block campaign of 1981, which saw Kieran Doherty elected as a TD for the Cavan–Monaghan constituency. Between 1982 and 1985, he was general manager of the republican newspaper "An Phoblacht". Ó Caoláin's first political success came in 1985, when he was elected to Monaghan County Council as a Sinn Féin County Councillor. In 1989, loyalist paramilitaries attempted to kill him and another Sinn Féin Councillor. At the 1984 and 1989 European Parliament elections he stood unsuccessfully in the Connacht–Ulster constituency.