Gillacommain mac Niall (died 991) was king of Uí Díarmata.
Gillacommain mac Niall was a member of the SíOl 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).
He was king in 991 but was at war with Cú Ceanain mac Tadhg, a son of his predecessor.
The war ended with their mutual deaths, and another unattested member of the Uí Díarmata, Muirgheas mac Aedh, became king.