Ó Caoláin represented Sinn Féin at the "Forum for Peace and Reconciliation" in Dublin in the mid-1990s. In 1996, he was an unsuccessful candidate in the Northern Ireland Forum election in Newry and Armagh. He was also a member of the Sinn Féin negotiations team during the talks which led to the Good Friday Agreement in 1998. Following the 1997 general election, he was elected to Dáil Éireann for the Cavan–Monaghan constituency, making him the first Sinn Féin TD elected since 1957, and the first Sinn Féin TD to take his seat at Dáil Éireann in Leinster House since 1922. He was subsequently re-elected at the 2002 general election and was joined by four other Sinn Féin deputies. He was re-elected at the 2007 general election, 2011 general election and 2016 general election. Ó Caoláin is currently Sinn Féin's Spokesperson on Health and Children. He accepts the average industrial wage and donates the remaining portion of his TD salary to his party.