He was the author of Turas na dTaoiseach/the Departure of the Lords, a diary of the Flight of the Earls, kept from September 1607 to November 1608, from Rathmullen, County Donegal to Rome.
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 in his immediate pre-flight year a resident of Portnelligan, Tynan, County Armagh, listed with six other Ó Cianáins as among the follower of Henry Óg Ó Neill in a pardon list 'fiant' of December 1602 (Cú Chonnacht, Seán mac Conchobhair, Pádraig Óg, Uaithne, Seán mac Pádraig Mhóir).
Tadhg's property—fifteen cows, eight calves, one garron, one hackney, twenty-five swine, all valued at £22.6s.3d—were forfeited but later returned to his wife at the intervention of the Earl of Thomond, who was a relative of hers.
In the aftermath of the departure, Ó Cianáin's property, valued at £22 6 shillings 2 pence, was seized, but later returned to his wife at the behest of Donogh O'Brien, 4th Earl of Thomond.