Domhnall's nickname 'An Saithnech' probably meant that he had been fostered in Saitne, now Dunsany, County Meath, by the Ua Cathasaigh clan.
On the death of the previous chief, his brother Matha Mág Tighearnán in 1311, Domhnall took the chieftaincy and resided in the castle of Croaghan of the Cups (Irish- Cruachan O'Cúbhrán), now in the townland of Coolnashinny, besides the modern town of Killeshandra.
Domhnall was murdered in 1312 by Cathal 'na-taisech' Ó Ruairc (d.1329), the same man who murdered his brother Matha Mág Tighearnán in the previous year, so Domhnall may have been trying to get revenge for Matha's death.
The Annals of Connacht for the year 1315 state- Domnall Mag Tigernain, chieftain of Tullyhunco, who used to be called the Saitnech, was killed by Cathal na Taisech O Ruairc.
The Annals of Loch Cé for the year 1315 state- Domhnall Mac Tighernain, dux of Tellach-Dunchadha, who was usually called 'the Saithnech', was slain by Cathal-na-taisech O'Ruairc.