On the death of the previous chief, his brother Conchobar ‘Buidhe’ Mág Tighearnán in 1314, Tomás 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.
Mag Shamhradhain went to the house of Ua Concobuir the same year and he came back again and on his return an attack was made by the Clann-Muircertaigh and by Muinter-Eoluis and by the muster of the Breifni, both Foreigner and Gallowglass, on him.
Magauran (i.e. Thomas) afterwards went to the house of O'Conor; but, on his return, the Clann-Murtough, and the Muintir-Eolais, assembled to meet him, and took him prisoner, after having slain many of his people.
Afterwards Mag Samradain went to O Conchobair's house, and as he was coming away the Clan Murtagh and the Muinter Eolais assembled to intercept him, captured him and killed many of his men.
Poem 25, stanza 20 states- He drove Teallach Dúnchadha from their land, valiant the host that marched on Cruacha, Fal’s prince in their midst.
Poem 26, stanza 13 states- Coming to fair Loch Uachtair, the three tuaths under the raider of Carn Currnán muster at Cruacha Ó gCubhrán.
The Annals of the Four Masters for 1346 state- Ivor, the son of Murrough O'Farrell, was slain by Brian Mac Tiernan and the Clann Murtough.
Tomás had five sons- Fergal Mág Tighearnán who later was chief of the clan, Domhnall, Mahon, Macraith and Cú Connacht.