 He was elected unopposed as a Sinn Féin MP for the Tipperary North constituency at the 1918 general election.In January 1919, Sinn Féin MPs refused to recognise the Parliament of the United Kingdom and instead assembled at the Mansion House in Dublin as a revolutionary parliament called Dáil Éireann, though MacDonagh did not attend as he was in prison.He was elected unopposed as a Sinn Féin Teachta Dála (TD) for the Tipperary Mid, North and South constituency at the 1921 elections.