Gall-Gaidhil warrior
Thomas made much of his career fighting in Ireland, starting from 1212 when, accompanied by sons of Raghnall mac Somhairle, he led a raid on the Cineál Eoghain city of Derry with seventy-six ships, sacking the city. In 1214, accompanied by Ruaidhrí mac Raghnaill, Thomas led another attack on Derry, sacking the city again and looting the church.
Tír Eoghain at the time had two rival kin-groups, the Inishowen-based Mac Lochlainn group and the Ó Néill based south of Sperrin Mountains. It is probable that the former were the main victims of Thomas's attack. Two years later, in 1216, Muireadhach mac Ailein, son of the mormaer of Lennox, defeated and killed the chief of an Inishowen group known as the Cineál Fearghusa, perhaps in support of Thomas's aims. During Thomas's 1212 raid on Derry, the men of Tír Conaill had simultaneously attacked the Cineál Eoghain in Inishowen.
Thomas's activities against Tír Eoghain may have been related to the attempts of the MacUilleim [MacWilliam] branch of the Scottish dynasty to gain the throne of Scotland. One of them, Gofraidh mac Domhnaill, had come from Ireland to Ross to renew a bid for the throne in 1211, and Thomas helped oppose him in the resultant royal campaign. There have been some connection between Meic Uilleim activities and Thomas's campaigns against Derry, with the Meic Uilleim perhaps being sheltered and supported by the Ó Néill, Aodh Méith.
Nonetheless, from 1210 King John had already set about trying to conquer Ulster west of the River Bann, and granted much of what are now counties Antrim, Londonderry and Tyrone to Thomas's brother Alan, the grant later being extended to include Thomas. Thomas himself was granted the core territories of Tír Eoghain in 1213, but the grant was "speculative" [Oram] and Thomas never became ruler of Tír Eoghain.
He did however, with English help, construct a castle at Coleraine, and the English appointed him keeper of the castle at Antrim in 1215. He appears to have travelled to England in 1219 in order to do homage to the new king of England, Henry III, but in 1221 was back in the region as he defeated and killed Diarmaid Ua Conchobhair, son of former Irish high king Ruaidhrí Ua Conchobhair; Diarmaid was returning to Ireland with a fleet he had raised in the Hebrides to help restore to the Connacht kingship the Ó Néill-backed anti-English Cathal Croibhdhearg Ó Conchobhair.
The Galloway lands in Ulster were threatened by Hugh de Lacy's return to the earldom of Ulster. The former earl of Ulster had previously fallen out of favour with the king, and suffered forfeiture. He returned to force his way back into power in Ulster and by 1227 had forced King Henry to acknowledge his claim to the earldom.
In the process Coleraine castle had been destroyed, and although they held on to much of their gains until at least 1226, it may be significant that when the castle at Coleraine was restored in 1228 it was by Earl Hugh and not Thomas. It is possible that Thomas's ability to manage his Irish possessions was hindered by his brother Alan's use of the Galwegian fleet to interfere in Manx succession disputes. As late as 1228 Thomas participated in Alan's invasion of the isle of Man, which installed Raghnall mac Gofraidh as king.