The origins of Christianity in Ossory are associated with St. Kieran whose foundation, Saighir Kieran, flourished for many centuries. St. Canice (c525–c599), founded a monastery at Aghaboe. St. Canice is thought to have established a cell in the vicinity of Kilkenny, thus bequeathing his name to the city, Cill Chainnigh, Canice’s Church. Kilkenny, the chief city and largest centre of population in the Diocese of Ossory, possesses the second largest mediaeval cathedral in Ireland, only St. Patrick’s Cathedral in Dublin being a larger mediaeval building.

The Diocese of Kilkenny was founded at the Synod of Rathbreasil in 1111, and was re-named Ossory shortly afterwards. The Diocese of Ossory was united with the Dioceses of Ferns and Leighlin under the Church Temporalities Act of 1834. The Dioceses of Ossory, Ferns and Leighlin were united with the Dioceses of Cashel, Waterford and Lismore by Act of the General Synod in 1976.