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,...