The first bishop of Killala mentioned in Roman records was Donatus O'Bechdha: his possession of the diocese was confirmed in a rescript dated 30 March 1198 by Pope Innocent III.

In the Middle Ages monasteries in the diocese included the three just mentioned, together with Rathfran, Ardnaree, Rosserk, Moyne, and Bofeenaun as well as other churches like Kilglass.

In the Reformation period, great efforts were made to establish English rule along the western sea board and conflict with religious authorities was part and parcel of that reality.

On a trumped-up charge, Bishop Redmund O'Gallagher, a thorn in the side of the authorities, was imprisoned and banished from the diocese.

Francis Kirwan, who was appointed in 1645, is the only Killala bishop who had a biography written of him, by his nephew, and it gives a good insight into the diocese in the middle of the seventeenth century.

Bellew was involved in the two great issues of his time, the founding of Maynooth College and the struggle for Catholic Emancipation.

In 1825 John MacHale, later Archbishop of Tuam, became coadjutor to Bishop Thomas Waldron who assigned to him a project first proposed in 1820: building a new cathedral to replace the old thatched church built about 1740.

In 1847 a Mayo road inspector reported that he had secured the burial of 140 bodies which he found lying by the wayside, while in the same year fourteen schooners left Westport laden with wheat and oats.

The Sligo Champion of 26 February 1847 reported, "Every hour the calamity is increasing, hundreds of unfortunate creatures have, within the last week, died of starvation.

After a short period of growth in the 1970s, when 20,000 emigrants returned to Connacht, rural communities in the West continued to decline.

[citation needed] Out of these meetings grew "core groups" which had input into an EU-funded study of the West of Ireland called by the bishops.

[citation needed] This process, generated by the bishops' initiative, resulted in a number of developments including: the establishment by the bishops of the Council of the West, a government task force which published the "Report on Crusade for Survival" and the establishment by the government on 1 February 1999 of the Western Development Commission.