Iserninus (d. 465/8), missionary bishop and church founder, was regarded by early Irish hagiographers as an associate of St Patrick (qv). His personal name, possibly derived from isarno- (iron), suggests a Gaulish origin. It may be significant that in Irish sources he is occasionally referred to as Epscop Fith, from the Latin episcopus vetus (the old bishop). Although not mentioned in surviving continental records, Iserninus features prominently in Patrician documents. Muirchú (qv), Tírechán (qv), and the Tripartite Life all repeat claims that Iserninus and other contemporaries were subject to Patrick, in order to bolster later claims by Armagh to jurisdiction over Gaulish foundations in Ireland. While the annals seem to provide independent testimony, asserting that in 439 Iserninus was sent to Ireland with Auxilius (qv) and Secundinus (qv) to assist Patrick, it must be acknowledged that such early entries do not represent contemporary chronicling, but were compiled at a later date with considerable input from Armagh.
The hagiographers maintain that at each stage of his career Iserninus was junior – if not expressly subject – to Patrick. Both, it is claimed, were students at Auxerre under Bishop Germanus, but Iserninus (and Auxilius) were ordained to lower clerical grades on the day that Patrick was conferred with episcopal orders. When Iserninus was dispatched to Ireland by Germanus, he sought to change course but was driven by the wind to his appointed destination. At first a reluctant missionary, Iserninus laboured in the district of Cliu (in east Co. Carlow). He is credited with having established churches at the unlocated sites of Toicuile and Ráith Foalascich, and at Láthrach dá Arad (Larah, near Ballon, Co. Carlow). Having secured the patronage of the sons of Cathbath, ancestors of the Fothairt, Iserninus was forced into exile by the king of Leinster, Énnae Cennselach (qv). Reprieved due to the intervention of Patrick, Iserninus is said to have yielded his churches to the saint. At this stage, it appears, Iserninus founded Áth Fadhad (Fithot) – the ‘ford of Fith/Fadhad’ (Aghade, Co. Carlow). Another tradition claims that Iserninus, along with Mac Táil (qv) was placed over the church of Kilcullen by Patrick.
The name of Iserninus, along with that of Auxilius, is attached to a motley collection of early synodal decrees which allegedly issued from the ‘first synod of Patrick’. Given the geographical focus of his career, it is more likely that Iserninus worked in association with Bishop Palladius (qv). Iserninus is included in the list of bishops preserved in the Book of Leinster. He died, according to the annalistic compilers, in 465 or 468.