O'NEILL, the name of an Irish family tracing descent from Niall, king of Ireland early in the 5th century, and known in Irish history and legend as Niall of the Nine Hostages.
He is said to have made war not only against lesser rulers in Ireland, but also in Britain and Gaul, stories of his exploits being related in the Book of Leinster and the Book of Ballymote, both of which, however, are many centuries later than the time of Niall.
The descendants of Niall spread over Ireland and became divided into two main branches, the northern and the southern Hy Neill, to one or other of which nearly all the high-kings (ard-ri) of Ireland from the 5th to the 12th century belonged; the descendants of Eoghan being the chief of the northern Hy Neill.'
Of this monarch, known as Murkertagh MacNeill (Niall), and sometimes by reference to his mother as Murkertagh Mac Erca, the story is told, illustrating an ancient Celtic custom, that in making a league with a tribe in Meath he emphasized the inviolability of the treaty by having it written with the blood of both clans mixed in one vessel.
His grandson, Niall (791-845), drove back the Vikings who in his time began to infest the coast of Donegal.