Ó Cianáin was a member of a professional Irish learned family who were originally erenaghs of the parish of Cleenish, Lough Erne, but who had served for several centuries as historians to Mag Uidir of Fear Manach (2007, p. 437).
Tadhg had at least one sibling, Cu Chonnacht Ó Cianáin, (died 1615), a rymer or chronicler to Rory Maguire.