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).
Among the other works copied and compiled in this period were Reim Rioghroidhe (Royal List) in 1630, Leabhar Gabhála (Book of Invasions) in 1631,[REF] Cogad Gáedel re Gallaib.