If he was indeed the patriarch, Niall of the Nine Hostages would rank among the most prolific males in history, behind Genghis Khan, ancestor of 16 million men in Asia, but ahead of Giocangga, founder of China's Manchu dynasty and forefather of some 1.6 million. This calculation, and the estimate of the I.M.H. signature's frequency in New York, were derived from a database of Y chromosome mutations.

The writer and actor Malachy McCourt said he was not surprised, since every Irish person is related to a king.

"They didn't mind who they slept with, and they had first dibs," he said. "It's so boring. It's not like the house of Windsor; every tribe had its own king."

He said Niall was "a highwayman. He was a slave trader, nothing noble about him. He was a pirate."

The link between the Niall Y chromosome and social power, which would have enabled the king to leave many descendants, "stretches back to the fifth century, which is a long time in Western European terms," Dr. Bradley said.

Asked if he himself carried the Niall signature, Dr. Bradley said he did and was "quite pleased," even though tradition holds that Niall captured and enslaved St. Patrick, who brought Christianity to Ireland.

Niall is said to have obtained hostages from each of the five provinces that then constituted Ireland, as well as from Scotland, the Saxons, the Britons and the Franks. He is thought to be the patriarch of the Ui Neill, meaning "the descendants of Niall," a group of dynasties that claimed the high kingship and ruled the northwest and other parts of Ireland from about A.D. 600 to 900.

But historians have tended to view the Ui Neill as a political construct, doubting their genealogical claims of descent from Niall and even whether Niall existed at all.