Grandson of Tuathal Ó Cléirigh, a chief of the
sept of
Uí Chléirigh in
Donegal, his exact place of birth in south Donegal is not recorded in surviving sources. He was baptised
Tadhg Ó Cléirigh, and was known by the nickname Tadhg an tSléibhe (meaning "Tadhg of the mountain"), but took the name of Mícheál when he became a member of the
Order of Friars Minor, commonly known as the Franciscan
friars. He was the youngest of four sons of Donnchadh Ó Cléirigh, and his mother was Onóra Ultach. Of his older brothers Uilliam, Conaire and Maolmhuire, Conaire is known to have worked on the annals as a scribe, while Maolmhuire also became a Franciscan friar at Louvain. Micheál was a cousin of
Lughaidh Ó Cléirigh (), also famous as an Irish historian and author of one of the major sources of the annals. As a member of one of the foremost learned families of
Gaelic Ireland, Ó Cléirigh received a wide-ranging and thorough education. He records that he was taught, for instance, by
Baothgalach Mac Aodhagáin, a learned cleric active in
County Tipperary, who became the
Bishop of Elphin. Tadhg followed Maolmhuire to continental Europe some time after the
Flight of the Earls. He may be the
Don Tadeo Cleri who was serving as a soldier in Spain in July 1621. At some point before March 1623 he became a
lay brother of the Franciscan Order, never becoming ordained a priest. ==Scholarship==