Tadhg mac Muircheartach was the first recorded king of Uí Díarmata, a local kingdom located in what is now north County Galway, Ireland, who died in 971.
Tadhg mac Muircheartach was a member of the Síl Muiredaig dynasty, who ruled as kings of The Connachta in what is now central County Roscommon.
Here a branch of the kindred seized territory and renamed it Uí Díarmata, after its founder, Diarmada Finn mac Tomaltaig, who was in turn a great-grandson of King Indrechtach mac Muiredaig (died 723).
Murchad Glun re Lar mac Flaithbertaigh, King of Ailech and King Cathal mac Tadg of Connacht fought each other at Ceis Corran in 971.
His son, Concenaind mac Tadhg, though never apparently king himself, would give his name to the ruling dynasty of Uí Díarmata, Ó Cú Ceanain (anglicised "Concannon")