A bishop of Mayo, probably Gille Ísa Ua Maílín, took the oath of fealty to King Henry II of England in 1172.
The bishopric in some way represented the lordship of Muirchertach Muimhnech Ua Conchobair taniste of Connacht, who died in 1210, and his family Clan Murtagh O'Conor who controlled the area up to the 1230s.
In 1216, Pope Innocent III heard the case in Rome, and gave sentence in favour of Tuam.
His sentence was maintained by papal legate James in 1221, and was finally confirmed by Pope Gregory IX on 3 July 1240.
The Roman Catholic see continued until the early seventeenth century, when, after a long vacancy, it was united to the archdiocese of Tuam in 1631.