He says the genetic match-up was also significantly higher in men bearing surnames linked to different branches of Uí Néill.
"Irish surnames are typically based on people who are the eponymous ancestors," said Bradley, whose own name claims ancestry to Niall.
Other examples include Boyle, Egan, Flynn, Gallagher, McGovern, McManus, Molloy, O'Connor, O'Reilly, and Quinn.
The approximate age of the Y chromosome was gauged by looking at genetic mutations that accrue over time. "Mutation occurs more or less like a clock," Bradley said. "The chromosome's age was consistent with someone of Niall's vintage, about 1,700 years old."
The same chromosome was also found in almost 17 percent of males tested in western and central Scotland and around two percent of New Yorkers of European origin.
The team estimates that two to three million males around the world share this same Irish ancestor.
The study suggests a link between powerful men and a strong genetic legacy, as more powerful men would have commanded access to more women.
"Polygamy was widespread, even in post-Christian Ireland," Bradley said. "Earlier Irish customs were quite resistant to change. Divorce was allowed, and concubines."
One 15th-century nobleman with Uí Néill lineage, Turlough O'Donnell, is known to have had 18 sons with 10 different women. His sons gave him 59 grandsons.
Other DNA studies centering on ancient leaders have produced similar results.
Genghis Khan, the Mongol emperor who conquered most of Asia in the 13th century, has nearly 16 million male descendants living today, according to a study published in the American Journal of Human Genetics in 2003.
His grandson, Kubilai Khan, who established the Yuan Dynasty in China, had 22 legitimate sons and was reported to have added 30 virgins to his harem each year.
Another study published last year suggests that 1.5 million Chinese men are directly realted to Giocangga, grandfather of the founder of the Qing dynasty.
Qing nobility were an elite class that had wives and concubines, according to Chris Tyler-Smith, a geneticist at England's Wellcome Trust Sanger Institute and a researcher for National Geographic's Genographic Project.
Because of the nobility's privileged status, their children would have had a good chance of survival, Tyler-Smith notes.
Dan Bradley says such genetic fingerprinting is potentially a very useful historical tool.
"It gives scientific support for medieval genealogies which I wouldn't necessarily have believed beforehand," he said.
"There's nothing written about Niall until hundreds of years after the time he would have lived, so you're talking about a character who was almost Arthurian," he added.
"There's a sort of credibility train that I think many people, including myself, would have jumped off before you got to him."
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