At the Synod of Kells in 1152, the archdiocese of Tuam was established with six suffragan dioceses.
The bishopric of Kilfenora was united to Tuam from 1661 to 1742, Ardagh from 1742 to 1839, and Killala and Achonry from 1834.
On the death of Archbishop Le Poer Trench in 1839, the Ecclesiastical Province of Tuam lost its metropolitan status and became the united bishopric of Tuam, Killala and Achonry in the Province of Armagh.
After an unsettled period in the mid to late sixteenth century, the Catholic archbishopric has had a consistent succession of archbishops.
In 1631, the Catholic bishopric of Mayo was formally joined to Tuam by papal decree.