The Dál gCais ([d̪ˠaːlˠ gaʃ]; English: Dalcassians) was 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.