Ossory had been Christianized long before this. St. Ciarán, the patron saint of the diocese, was born about the 4th century at a place now known as "St. Ciarán's Strand", near Cape Clear Island, and was probably converted to the Catholic faith by foreign traders. According to tradition, he travelled to Rome and was there ordained priest and bishop. Having met St. Patrick, St. Ciarán received from him a bell with the charge to return to Ireland and there establish a monastery on the spot where the bell should first sound. When the saint had passed beyond Ossory, and was descending the )western slopes of Slieve Bloom, the bell at length sounded; and here St. Ciarán established the monastery of Seir-Kieran (now Saighir, Offaly), the centre from which Ossory was evangelized. St. Patrick also visited Ossory and preached and founded churches there. There is some difficulty in accepting the story of St. Ciarán having preached before St. Patrick, since the former is said to have flourished in the 6th century. It is, however, certain that St. Ciarán laboured in Ossory.
