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. In 1632 the group began to assemble the most extensive set of Irish annals ever compiled. The project took four years and resulted in the vast collection dubbed Annála Ríoghachta Éireann (Annals of the Kingdom of Ireland) but now better known as the Annals of the Four Masters. The 'four masters' in question are Mícheál Ó Cléirigh, Cú Choigcríche Ó Cléirigh, Fearfeasa Ó Maol Chonaire and Peregrine Ó Duibhgeannain and the term was devised by John Colgan. However, other important collaborators included Muiris mac Torna Uí Mhaolchonaire, and Ó Cléirigh's brother, Conaire. The work was completed in August 1636 and two manuscript copies of the annals were made. Ó Cléirigh had an interest in Irish Lexicography and compiled a well-known glossary. This was printed during the author's lifetime - in 1643 - as , (A New Vocabulary or Glossary). It has since been often known by the descriptive title , (in English: Michael O’Clery's Glossary). These two works are valuable for the etymological and encyclopaedic information contained in them. Among the other works copied and compiled in this period were: the medieval Irish account of clashes with the Vikings, Cogad Gáedel re Gallaib, twice, (in 1629, and again in 1636); the royal genealogy, Réim Ríoghraidhe in 1630; and Leabhar Gabhála (Book of Invasions) in 1631. He subsequently produced his Martyrologium of Irish saints, based on various ancient manuscripts, such as the Martyrology of Tallaght. Later life and legacy He returned to the continent in early 1637. The only work by Ó Cléirigh to be published in his lifetime was his glossary of 1643. His precise date of death is unknown, but he is generally thought to have died at Louvain in 1643. Mícheál Ó Cléirigh appears as an historical character in Darach Ó Scolaí's novel, An Cléireach. In 1944, An Post issued two