Tomás Ó Cellaigh was a Catholic Bishop of Clonfert. His death is recorded as 6 January 1263.
O Cellaigh was elected to that position sometime prior to 7 November 1259, as he received possession of temporalities

Temporalities

Temporalities are the secular properties and possessions of the Christian Church. It is most often used to describe those properties that were used to support a bishop or other religious person or establishment. Its opposite description would be the spiritualities.In the Middle Ages, the...

 commencing on that date.

His name is also recorded as Tomás mac Domnaill Móir Ó Cellaig. He was probably from Ui Maine.