Killaloe Diocese comprises the greater part of County Clare, a large portion of Tipperary, and parts of King’s and Queen’s Counties, Limerick, and Galway. Its Irish name is Cill-da-Lua, so named from St. Lua, an abbot who lived about the end of the sixth century, and whose oratory can still be seen in Friar’s Island, near the town of Killaloe. At the Synod of Rathbresail in the first quarter of the twelfth century, Killaloe assumed its present shape, which is almost coterminous with the boundaries of the ancient Kingdom of Thomond. Ancient Thomond included the territories of Dal gCais (O’Briens, et al), Aradh, Eile and Muscraige Tire in the eastern portions, and Corco Baiscinn in the southwest.

Prior to the Synod of Rath-Breassail the diocese of Killaloe did not include the old Sees of Roscrea and Inniscathy (Inniscattery). By the late 12th century the ancient bishopric of Roscrea was united to the See of Killaloe. The Diocese of Roscrea was coextensive with the territory of the O’Carrolls (Eile), along with that of the O’Kennedys. The Diocese of Inniscathy, comprised the Baronies of Moyarta, Clonderlaw, and Ibricken, in Clare; the Barony of Connello, in Limerick; and in Kerry, the ancient region of the Uí Fidgente. The parish of Seir Kieran in King’s County, though in Thomond, was allowed to remain subject to the Diocese of Ossory, out of respect to the memory of St. Kieran.