The Dalcassians (Irish: Dál gCais [d̪ˠaːlˠ gaʃ]) were a Gaelic Irish tribe, generally accepted by contemporary scholarship as being a branch of the Déisi Muman, that became a powerful group in Ireland during the 10th century. Their genealogies claimed descent from Cormac Cas, who is said to have lived in the 3rd century AD. Their known ancestors are the subject of The Expulsion of the Déisi tale and one branch of their blood-line went on to rule the petty kingdom of Dyfed in Wales during the 4th century; probably in alliance with Roman emperor, Magnus Maximus.

Brian Bóruma is perhaps the best known king from the dynasty and was responsible to a significant degree for carving out their fortunes. The family had built a powerbase on the banks of the River Shannon and Brian's brother Mahon became their first King of Munster, taking the throne from the rival Eóganachta. This influence was greatly extended under Brian who became High King of Ireland, following a series of conflicts with Norse and other Irish tribes, before dying famously at the Battle of Clontarf in 1014. Following this the Dál gCais provided three more High Kings of Ireland; Donagh O'Brien, Turlough O'Brien and Murtagh O'Brien; but lost out to the longer established dynasties.

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