Domnall mac Caustantín is thought to have been king of Dál Riata in the early ninth century.
Domnall's existence is uncertain, and is based on attempts to reconcile eleventh century works such as the poem Duan Albanach and the Synchronisms of Flann Mainistrech with the evidence of the Irish annals.
[3] The only person of that relatively uncommon name known is Causantín mac Fergusa, king of the Picts from 792 to 820.
Since Áed Find died in 778, and his brother Fergus mac Echdach was king of Dál Riata at his death in 781, it is thought unlikely that Caustantín's son could have been king as early as 781.
Additionally, a king named Donncoirce is reported to have died in 792, and Conall mac Taidg died in 807, making it very difficult to accommodate a 24-year reign at this time.