This paper explores the different ways in which the career of Carthach – Carthagus in Latin, also known as Mochuda or Mo-Chutu – was remembered in both Latin and Irish accounts, in particular his expulsion from Rahan, where he was abbot for forty years, and his re-establishing that community at Lismore, where he died c.637. It argues that the Life of Carthach legitimizes his foundation at Lismore, by celebrating its initial establishment at Rahan, and by connecting him through a chain of prophecies to Íte, Brendan, Comgall, and above all Colum Cille. In this Life, Patrick is never mentioned, quite unlike the Life of Declán, which claimed Ardmore as the original bishopric of the Déise region. Carthach's expulsion from Rahan, provoked by loss of political support from local princes, took place just as other abbots of the region were hesitating to adopt the Roman date of Easter, in the manner of much of southern Ireland. Carthach, part of a group sympathetic to Gregory the Great and Roman practice, was perceived by his admirers as being connected to a line of saints that helped define the particular identity of the church at Lismore.