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.

Later offshoot septs of the dynasty included the families of MacDermot Roe.

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.

Pedigree:
https://www.libraryireland.com/Pedigrees1/macdermot-1-heremon.php