Carthach (Mochuta) (d. 637), monastic founder, was a son of Fínall and member of the Ciarraige Luachra group (Co. Kerry). He became abbot of the monastery of Rahan in Uí Néill territory (Co. Offaly), whence he was expelled in 636 (AU; Ann. Inisf. 638) and went south to Déisi Muman to found the monastery of Lismore, though he may have left some ‘cells’ of monks behind him in Lemanaghan and Clonfertmulloe. The reason for his expulsion is not precisely known, though the Annals of Tigernach (Tigernach Ua Bráein (qv)) tell us that it happened in diebus paschae (at the time of the Paschal controversy). It is quite likely that Carthach was an advocate of the Roman dating of Easter, but it is more likely that he was expelled for reasons of ecclesiastical or tribal rivalry.
The Latin and Irish Lives of Carthach were composed in the twelfth century or the early thirteenth century; though of considerable intrinsic interest they are not of immediate historical value in relation to Carthach. They describe his expulsion from Rahan, together with the monks and nuns from his former double monastery and the lepers from its leper colony, and the four stages of his journey to Lismore. There, he was granted a piece of land on the south bank of the river Blackwater on which to found his new monastery, on a site later occupied by Lismore castle. The eighth-century Irish tract on his expulsion from Rahan, ‘Indarba Mo-Chuda a rRaithin’, gives a vivid account of this event, but is of uncertain historical value.
It is likely that Carthach was the author of the abecedarian hymn to Camelacus (qv), fifth-century (?) founder of Rahan, found in the Antiphonary of Bangor. Some manuscripts credit him with a metrical rule in Irish, which consists of a series of precepts on the Christian duties of the various classes of society; being of ninth-century date, however, it is unlikely to be his work. Lismore subsequently became a centre for the study of Latin literature and canon-law. The Irish Augustine (Augustinus (Pseudo-) (qv)) dedicated his tract ‘De Mirabilibus Sacrae Scripturae’ to the ‘most venerable bishops and priests of the cities and monasteries, most especially of the Carthaginenses’, which was formerly thought to relate to Lismore, the city of Carthach, but has recently been interpreted as referring to Inis Cathaig (Scattery Island), the foundation of St Senan (qv) in the Shannon estuary, taking Carthaginenses to be a corruption of Cathagenses. Some early texts of the Irish canon-law collection, the ‘Collectio canonum Hibernensis’, compiled within the first quarter of the eighth century, were also produced at Lismore. In the parallel table of saints in the Book of Leinster (370a), ‘Mochuta Lismóir’ is paired with ‘Cyprianus Carthagenensis’, probably due to the similarity between the names Carthach and Carthage. Carthach's feast-day is 14 May.