The Diocese of Ossory is a diocese in Ireland in the Ossory region, established in the fifth century with the mission of St. Ciarán of Saighir, the borders of which were permanently set at the Synod of Ráth Breasail om 1111 AD. Its territory corresponds approximately to the medieval Kingdom of Ossory. The Roman Catholic Diocese of Ossory continues as an independent diocese within the Roman Catholic Church headed by the Bishop of Ossory; in the Church of Ireland, it is grouped with Cashel, Waterford, Lismore, Ferns and Leighlin in the Diocese of Ossory and Cashel. The Church of Ireland diocesan boundaries do not reflect Ossory's historical limits. Its primary patron saint is the first bishop of Ossory, Ciarán of Saighir; along with St. Cainnech of Aghaboe as a second patron.