The saint is mainly known as the tutor and fosterer of his greater namesake, Saint Carthage of Lismore, also known as Saint Mochuda. Carthage was of the Eóganacht Chaisil and son, or, more probably, grandson of Óengus mac Nad Froích whom Saint Patrick baptized. He was himself a disciple of Saint Ciaran of Clonmacnoise who had condemned him to a condign penance for a sin of the flesh committed in his younger days. On completion of his canonical penance Carthage was reinstated as a member of the religious brotherhood. Afterwards he founded the monastery of Druim Fertain and another monastery in the upper island of Lough Sheelin, County Meath.
In the barony of Clanmaurice is a townland called Monument on which are some scant remains of an ancient church called Cill Cartaig (Carthage's Church).