The progenitor of the family was Dermot mac Tadhg Mor, 7th King of Moylurg, who reigned from 1124 to 1159. He was a vassal and kinsman of the Ó Conchubhair, Kings of Connacht; their common ancestor was Tadg mac Cathal, King of Connacht from 925 to 956. They were based at McDermott's Castle, Lough Key.
Moylurg ceased to exist as a kingdom in the late 16th century, though the senior line of the MacDermot's continued to live a sometime poverty-stricken and precarious existence despite land confiscations and the oppression of the Penal Laws. During this era they were popularly accorded the title Prince of Coolavin; the current incumbent is Rory MacDermot, who succeeded his father in 2003.