Diarmait mac Tommaltaig (died 833) was a king of Connacht from the Uí Briúin branch of the Connachta.
[1] The death of his father Tommaltach mac Murgail (died 774) is recorded in the annals where he is called king of Mag nAi.
Then in 818 he imposed his authority over the Uí Maine (the third major grouping of Connacht in Counties Galway and southern Roscommon) with a Pyrrhic victory at the Battle of Forath in the territory of the Delba Nuadat (a subject people of Uí Maine in southern Roscommon).
In 829 the high king Conchobar mac Donnchada (died 833), with the men of Mide (Meath), attacked Connacht and defeated the Connachta in battle.
[8] In 830, the King of Munster, Feidlimid mac Crimthainn (died 847), invaded and crushed the southern Uí Briúin Seola.