Mayo became a diocese in 1152, following the Synod of Kells, controlling the part of County Mayo now under the jurisdiction of the Roman Catholic Archdiocese of Tuam.

Following the Norman invasion the general area came under the control of Maurice De Prendergast who's main stronghold was at Brize, in the east of Mayo Abbey parish.

A Norman town was founded at Mayo and an Augustinian Abbey was built on the site of the original monastery c.1400 AD., the ruins of which are still visible today.

The town was of sufficient size and importance by the end of the sixteenth century to give its name to County Mayo, during the Composition of Connacht under Sir Henry Sidney in 1595 AD.

In 1617 the Abbey was sacked during the reign of Queen Elizabeth the first and it's lands were confiscated. The diocese was merged with Tuam in 1630. Mass continued to be celebrated in the ruins of the Abbey during penal times.