Cellach of Armagh was a twelfth-century Irish prelate who served as the Archbishop of Armagh. Born into the Clann Sínaig, a family that had held the abbacy of Armagh for generations, he broke with tradition by seeking holy orders rather than remaining a layman. This significant step helped transition the Irish church from a monastic system toward a diocesan structure. Following the Synod of Rathbreasail, he became the first metropolitan primate of Ireland, asserting the authority of Armagh over the newly organized ecclesiastical provinces.