From An ecclesiastical history of Ireland: from the introduction of Christianity into that country, to the year 1829, by Michael John Brenan, Volume 1 (1840), from p. 149:
The See of Ferns was founded by St. Aidan or Maidoc about the year 600. Aidan was of an illustrious family in Connaught his Father Letna having been descended from Brian Prince of the Hy bruin sept in Breffny and his mother from the high and ancient race of Auli.(1.) When a youth Aidan was one of the hostages whom the people of Breffny had been obliged to give to Anmiracus King of Ireland and some time after his liberation he withdrew from his native country and retired to the establishment of St David at Menevia in Wales. Here his extraordinary sanctity soon rendered him celebrated. About the year 589 Aidan departed from Menevia and having landed in the now County of Wexford he erected a Church at a place called Ardlathran in the southern part of that county His next establishment was at Clonemore in the barony of Bantry and having been held in great reverence by Brandubh King of Leinster that Prince assigned him a site on which he erected his celebrated Monastery of Ferns about the year 600. At the request of Brandubh a numerous Synod was soon after convened in which it was decreed that Ferns should become an Episcopal See and be moreover raised to the dignity of Archbishopric of Leinster On this occasion Aidan was consecrated its first Bishop. (2.) Usher remarks that by this decree the Archiepiscopate of Leinster had been removed from Sletty but was afterwards transferred from Ferns to Kildare. (3.) It is at all events most certain that these so called Archbishops whether of Sletty or of Ferns were not strictly speaking Metropolitans nor were they invested with Archiepisoopal power or that jurisdiction provided by the Canon law. They enjoyed by courtesy and very often through the favour of princes a degree of honorary pre-eminence and for this reason we find the title passing in those days from one see to another. The reputation of St. Aidan was not confined to Ireland. His memory has been highly revered both in Wales and in other countries and several miracles have been attributed to him. He died on the 31st of January AD 632 and was buried at Ferns. (4.) St. Aidan was succeeded in the See of Ferns by St. Moling, a native of the territory of Hy Kinsellagh. Between the death of this Prelate and the incumbency of the learned Alban O Mulloy in the twelfth century the names of fifteen Bishops have been recorded while their acts like most of our other national documents have perished beneath the fury of the Danes or the still more unsparing rapacity of the English invaders.
(1.) AASS p. 216
(2.) Vita c. 28
(3.) Usher p. 965
(4.) Usher p. 966