Tadhg Ó Cellaigh (died 1316) was King of Uí Maine and Chief of the Name.
Tadhg was one of five sons of Domhnall mac Conchobar mac Tadhg Taillten Ó Cellaigh, listed as
"Gilbert, King of Hy-Many, David, Tadhg Mor of the Battle of Ath na Righ, and Conchobhar, King of Hy-Many, and Aedh. Only two of them were by the same mother, viz., Tadhg and Conchobhar, and their mother was Abis, the daughter of O'Flainn O'Flynn."
Tadhg usurped his brother, Gilbert Ó Cellaigh in the wake of the war of 1315–16, which arose with the return of Ruaidhri mac Cathal Ó Conchobair from Ulster and his successful seizure of the kingship of Connacht from his cousin, Fedlim Ó Conchobair. Tadhg expelled Gilbert and allied with Ruaidhri, who
assembled the men of Connacht and Brefne, together with many gallowglasses, and penetrated to the midst of Sil Murray and the rest of Connacht, and forthwith burned the town of Sligo, Ballymote, the great castle of Kilcolman, Ballintober, Dunamon with its castle, Roscommon, Randoon and Athlone, as well as all the houses which lay on his route ... he took pledges and hostages of the rest of the Sil Murray and was made king on Carnfree. After this he remained for some time among them, ravaging every neighbour who was not submissive to him.
Tadhg accompanied Ruaidhri on his subsequent winter raids around Connacht, such as those at
Leith Luigne and the slopes of Sliab Gam, and in particular to Glenn Fathraim, where they killed many thousand cows and sheep and horses. At that time they stripped women and ruined children and lowly folk, and never within the memory of man were so many cattle, fruitlessly destroyed in one place.
Returning to Ui Maine, he proceeded to burn the castle of Aughrim, County Galway and plunder and burn Máenmaige.