Born in Cavan, O'Reilly settled in Dublin where he undertook the compilation of the work for which he is best remembered, his Irish-English Dictionary published in 1817. He was appointed assistant secretary to the Iberno-Celtic Society on its foundation the following year with the purpose of preserving and promoting Irish literature. His work during the 1820s included a Dictionary of Irish Writers, catalogues of Irish manuscripts in Dublin libraries including Trinity College, and Irish nomenclature in Ordnance Survey maps. He died in 1830.