From Science:

Legend has it that, while raiding England around 500 C.E., the Irish warlord Niall of the Nine Hostages took a young St. Patrick prisoner and brought him to Ireland. Historians disagree about whether Niall was really the kidnapper, but one thing is for certain: This ancient king went on to found the most powerful ruling dynasty in Irish medieval history, the Uí Néill (literally “descendants of Niall”). Now, a study reveals that this royal lineage may be imprinted in the genes of roughly a tenth of Irish men living today.
Although most of our genetic makeup comes from both parents, men inherit their Y chromosome exclusively from their fathers. As such, geneticists can trace paternal lineage by studying genetic markers on this chromosome. Small mutations creep in over generations, so the number of differences in the markers of any two men can reveal how long ago their common forefather lived. In 2003, a population study in East Asia discovered a set of Y chromosomes with very similar markers among men living in what had been the Mongol Empire. The scientists speculated that some 16 million men with these markers were all descendants of Genghis Khan (Science, 23 February 2003, p. 1179).

Obviously, there’s more.