He rejected the Anglo-Irish Treaty and fought in the Anti-Treaty IRA during the Irish Civil War of 1922–23. He was one of the senior IRA figures in Kerry during this conflict, under the command of Humphrey Murphy. In the war's early months, he commanded a Kerry column in the fighting in Limerick city and at the Battle of Kilmallock, before retreating back into Kerry and pursuing guerrilla warfare. In January 1923, he, along with John Joe Sheehy, led an attack on the National Army barracks at Castlemaine, using an improvised mortar.