Breen was elected to Dáil Éireann at the 1923 general election as a Republican anti-Treaty Teachta Dála (TD) for the Tipperary constituency. Following the Anglo-Irish Treaty, Breen joined the Anti-Treaty IRA in the Civil War, fighting against those of his former comrades in arms who supported the Treaty. He was arrested by the National Army of the Irish Free State and interned at Limerick Prison. He spent two months there before going on hunger strike for six days, followed by a thirst strike of six days. Breen was released after he signed a document pleading to desist from attacking the Free State.