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.'
O'Neill, grandson of Neill, or Niall, the name O'Neill becoming about this time an hereditary family surname 2), whose grandson, Flaherty, became renowned for piety by going on a pilgrimage to Rome in 1030.
Neill Mor's great-greatgrandson, Henry O'Neill, was created baronet of Killeleagh in 1666.
His son, Sir Neill O'Neill fought for James II.
Through an elder line from Neill Mor was descended Brian Mac Phelim O'Neill, who was treacherously seized in 1573 by the earl of Essex, whom he was hospitably entertaining, and executed together with his wife and brother, some two hundred of his clan being at the same time massacred by the orders of Essex.