Ó Caiside's family was of Ulster stock, his parents settling in Drishacaun townland, parish of Kilmurry, Castleplunkett, County Roscommon.
What is known of his life is told in Eachtra Thomáis Uí Chaiside ("The Adventures of Tomás Ó Caiside") which he wrote himself.
Two surviving copies, one by his friend and contemporary Brian Ó Fearghail, are kept in the Royal Irish Academy and the British Museum.
He was dismissed from the friary of Ballyhaunis "on account of a bad senseless marriage" and spent the rest of his life travelling all over Ireland and Britain, as well as central Europe.
He mentions having been in the Electorate of the Palatinate; the Black Forest; Sandhausen; Hanover; Prussia; Brunswick; Bristol; and Bideford.