The diocese of Kilmore was originally called Breifne (Latin: "Tirbrunensis", "Tybruinensis" or "Triburnia"; Irish: "Tír mBriúin", meaning "the land of the descendants of Brian", one of the Uí Briúin kings of Connacht). It was formally established as one of the dioceses approved by Cardinal Giovanni Paparoni at the synod of Kells in 1152. The boundaries of the diocese are approximately the same as those of the ancient Kingdom of Breifne; which today consists of almost all of County Cavan, roughly the northern half of County Leitrim, and parts of County Fermanagh, County Meath and County Sligo.