This was the ancient territory of the kingdom of Uí Maine (Hy-Many), as it existed when the diocese was formed.

It was also deeply involved in the eighth century spiritual reform movement of the Céli Dé.

[citation needed] Saint Brendan's fame as a seafaring missionary contributed to its pre-eminence in later times and led to its choice as an episcopal see in the twelfth century.

[citation needed] In 1170, the Annals of Clonmacnois record that "there was a great convocation of the clergy of Ireland at Clonfert by commission from the Pope for the reformation of certain abuses of a long time used in Ireland", which was presided over by Saint Laurence O'Toole as papal legate.

Landlord intransigence prevented the building of a cathedral in Loughrea until 1897 when Bishop Healy laid the foundation stone.

The era of the Celtic Revival and Irish Stained Glass had begun, and influenced its interior decoration.