The Book of Caillin was written in Irish between 1350 and 1400, however, the location of the original manuscript is unknown. For the most part, it deals with the life, virtues and wondrous deeds of Caillin and was filled with fantastic stories, wild anachronisms and extravagant claims on behalf of Caillin. It gave a great insight into the legends and genealogies of the Conmaicne, the Irish tribe of which Caillin was the most renowned member.
There were also many poems, one of which is 816 lines long, historic, prophetic and attributed to St. Caillin. In 1516 the Coarab of Fenagh, Tadhg O’Rodaighe, had the book transcribed and re-cast. Additional extracts of poems and prose were included to become the Book of Fenagh as it is now known. The text is not confined to fact alone but the scribing was adapted to best suit the interest of Fenagh monastery. The Book of Fenagh was used by persons wishing to swear oaths in order to clear themselves of crimes that they had been accused of or if they wanted to make a specially binding contract.