His mother, Eithne Imgel, daughter of the king of Alba (originally meaning Britain, later Scotland), was pregnant when Fíachu was overthrown, and fled to her homeland where she gave birth to Túathal.
Twenty years later Túathal and his mother returned to Ireland, joined up with Fiacha Cassán and Findmall, and marched on Tara to take the kingship.
The Annals of the Four Masters[7] features a similar revolt a few generations earlier, led by Cairbre Cinnchait, against the High King Crimthann Nia Náir.
On this occasion Crimthann's son Feradach Finnfechtnach is the future king who escaped in his mother's womb, although the Annals claim he returned to reclaim his throne only five years later.
He has Crimthann hand the throne directly to his son, Feradach, and makes Cairbre Cinnchait, whose ancestry he traces to the Fir Bolg, the leader of the revolt that overthrew Fíachu, killing him at a feast.
Túathal, or his wife Baine, is reputed to have built Ráth Mór, an Iron Age hillfort in the earthwork complex at Clogher, County Tyrone.
The scholar T. F. O'Rahilly suggested that, as in many such "returned exile" stories, Túathal represented an entirely foreign invasion which established a dynasty in Ireland, whose dynastic propagandists fabricated an Irish origin for him to give him some spurious legitimacy.
[9] Professor Dáithí Ó hÓgáin reckoned his Celtic name was Teutovalos ('tribe-ruler') and he was a great leader of the northern branch of the Venii tribe, or the 'people of Condos' who overthrew the kingship of the Lagini at Tara.
When the genealogies were written a few centuries later his name was noted as 'Tuathal', and the epithet teachtmhar, a Celtic compound meaning 'appropriator of wealth' was added referring to his followers large-scale raids on the British coast.