He was second-in-command to Thomas Ashe (who would later die on hunger strike) in an encounter with the armed Royal Irish Constabulary (RIC) at Ashbourne, County Meath during the Easter Rising in 1916—one of the few stand-out victories won by republicans in that week, and generally credited to Mulcahy's grasp of tactics. In his book on the Rising, Charles Townshend principally credits Mulcahy with the defeat of the RIC at Ashbourne, for conceiving and leading a flanking movement on the RIC column that had engaged with the Irish Volunteers. Arrested after the Rising, Mulcahy was interned at Knutsford and at the Frongoch internment camp in Wales until his release on 24 December 1916.