Maghnus was a son of King Tairrdelbach Ua Conchobair (1088-1156) and one of his six wives.
Maghnus and his brother, Brian Luighnech Ua Conchobhair, were killed at the battle of Crich Coirpre in County Sligo by King Flaithbertaigh of Tyrconnell.
The battle occurred "on the Saturday before Whitsuntide", "Sixteen of the sons of the lords and chieftains of Connacht were slain by the Kinel Connell, as well as many others, both of the nobles and the plebeians".
398–99), which Jaski identifies as Mac Magnusa of Tir Tuathail (EIKS, p. 316).
Today, some families named McManus, MacManus, McMannes, Mannus, Mannis and MacManners trace their heritage back to the sons of Maghnus.