Ó Cléirigh had already gained a reputation as an antiquary and student of Irish history and Irish literature, when he entered the Irish College of St Anthony at Louvain (Dutch: Leuven). In 1624, through the initiative of Aedh Buidh Mac-An-Bhaird (1580–1635), warden of the college, and himself a famous Irish historian and poet, and one of an old family of hereditary bards in Tyrconnell, he began to collect Irish manuscripts and to transcribe everything he could find of historical importance. To do this he returned to Ireland in 1626 and spent over a decade based at a Franciscan house by the River Drowes on the Donegal-Leitrim border. He was assisted by other Irish scholars, most notably Cú Choigcríche Ó Cléirigh, Fearfeasa Ó Maol Chonaire and Peregrinus Ó Duibhgeannain. Ó Cléirigh travelled widely throughout Ireland during this period, collecting and transcribing a vast quantity of Irish texts. His initial focus was material of ecclesiastical importance, particularly saints' lives, but by 1631 he and his colleagues were beginning to copy secular material such as the Irish pseudo-history "Leabhar Gabhála".