The foundation of the diocese dates from the time of St. Patrick, who placed his disciple St. Muredach over the church called in Irish Cell Alaid. In a well that still flows close to the town, beside the sea, Patrick baptized in a single day 12,000 converts, and on the same occasion, in presence of the crowds, raised to life a dead woman whom he also baptized. Muredach is described as an old man of Patrick's family, and was appointed to the Church of Killala as early as 442 or 443. His feast-day is on 12 August. It is probable that he resigned his see after a few years, and retired to end his life on the lonely island in Donegal Bay which has ever since borne his name, Innismurray. At Killala Patrick baptized the two maidens whom he met in childhood at Focluth Wood by the western sea, and whose voices in visions of the night had often pathetically called him to come once more and dwell amongst them. He came, baptized them and built them a church where they spent the rest of their days as holy nuns in the service of God.
Little or nothing is known of the successors of Muredach in Killala down to the twelfth century. Of the sainted Bishop Cellach, for example, we learn merely that he came of royal blood, flourished in the sixth century and was foully murdered at the instigation of his foster-brother. His name is mentioned in all the Irish martyrologies. Beyond doubt the most illustrious of them all belongs to modern times. With pride the people of Killala recall that John MacHale, Archbishop of Tuam, was a child of their diocese, and served his 'apprenticeship' as bishop amongst them. He was born at Tubbernavine at the foot of Mount Nephin on 6 March, 1791; became Coadjutor Bishop of Killala in 1825, bishop in 1834, and later in the same year was transferred to Tuam, where for nearly half a century he exercised a more potent influence on the civil and ecclesiastical history of Ireland than perhaps any of his contemporaries, with the single exception of O'Connell. He died on 7 November, 1881, and is buried in the sanctuary of the Tuam cathedral. After him came Doctor Finan, a Dominican priest of remarkable piety and attainments, but rather unfit, owning to his continental training, to direct the affairs of an Irish diocese. On his resignation in 1838, a parish priest of the Archdiocese of Tuam, Rev. Thomas Feeney, who had formerly been professor and president of St. Jarlath's College at Tuam, was chosen for the task of repairing the injury that ecclesiastical discipline had suffered during his reign. Feeney is said to have been a most happy selection under the circumstances. Thirty-five years of his firm and resolute rule obliterated practically all traces of the wretched controversies that distracted the diocese under his predecessor.
Along the left bank of the river are the ruins of several monasteries. Rosserk, a Franciscan house of strict observance, was founded in 1460. The beautiful Abbey of Moyne still stands nearly perfect on a most picturesque site just over the river, and further on, north of Killala, was the Dominican Abbey of Rathfran, also delightfully situated. On the promontory of Errew running into Lough Conn another monastery existed as such till comparatively recent times. A fine round tower in Killala itself, still in perfect preservation, indicated the ancient celebrity of the place as an ecclesiastical centre. It may be safely stated that in no other portion of Ireland of equal extent were the labours of St. Patrick and the holy founders of religious institutions who came after him so arduous and full of interest as in this beautiful district of Tirawley.
Tireragh and Tirawley need not rely entirely for their fame on the traditions of the remote past. Under learned early 20th century bishop Dr. Conmy the Catholic religion has made quiet but very gratifying progress. In Maynooth he held a distinguished place in the class that produced such men as the Cardinal Primate of Ireland and Archbishop Carr of Melbourne. After several years of fruitful labours as professor and missionary priest he was called in 1892 from the parish of Crossmolina to wield the crosier of Muredach. His rule has been characterized by prudence, and justice that is well tempered with mercy. Amongst his most conspicuous services to the twin cause of religion and education must be reckoned the building and equipping, from funds raised almost exclusively from his own faithful priests and people, of the splendid seminary in the town of Ballina and bids fair to revive the olden name of the School of Killala founded by St. Patrick.
Killala also boasts a Church of Ireland Cathedral, a striking and noteworthy stone building dedicated to St Patrick.