Very early on, the Irish developed patronymic surnames. Modern genetics can be used (misused) to test those genealogies. In the case of someone supposed to have lived 1500 years ago, Niall of the Nine Hostages, a y-chromosome appears in about 1/5 of the men of the O’Niell territory in the North. Brian Boru, 500 years later, does not dominate the O’Brians at all as strongly. It’s all a matter of time, statistics, and fast horses.

irishtimes.com