Niall of the Nine Hostages, a High King of Ireland who flourished early-to-mid 5th century A.D., and whose raids on the coastline of Britain are conventionally credited with bringing Saint Patrick to Ireland as a captive slave, is the most likely source of Y-chromosomal DNA found by scientists to be shared by one in twelve Irish males, and an estimated 3 million men world-wide.

Reuters report, and abstract of paper (subscribers only) in the American Journal of Human Genetics.

New York Times