In a PBS series on African-American ancestry that he hosted in 2008, Gates discovered his Irish roots when he found he was descended from an Irish immigrant and a slave girl.
He went to Trinity College in Dublin to have his DNA analyzed. There he found that he shared 10 of the 11 DNA matches with offspring of Niall of the Nine Hostages, the fourth century warlord who created one of the dominant strains of Irish genealogy because he had so many offspring.
Ironically, James Crowley, whose name in Gaelic means "hardy warrior," is also descended from the same line as Gates, having very close links to Niall of the Nine Hostages.
So the two men who took part in what is now an infamous confrontation outside the Gates home near Harvard this month are actually related through common Irish lineage -- one of the more extraordinary aspects of the incident that has sparked worldwide headlines.
Gates is one of many famous African-Americans with Irish heritage, including President Barack Obama and award-winning author Alice Walker.
On the PBS series, Gates visits Trinity College to find his roots, and says to the genealogist, "Do I look like an Irishman to you? I'm here to find my roots. I've been looking all over Africa and I couldn't find anybody, so I ended up here.
"I'm descended from a white man, he says. "A white man who slept with a black slave. And we know from the analysis of my DNA that ... goes back to Ireland. So maybe you can help me."
When the genealogist tells him he does indeed have Irish links, Gates says, "I find this oddly moving. It is astonishing," he says, "that I have a kinship with someone (Niall of the Nine Hostages) dating back to the fourth century A.D."
Irish American Descendants
Millions of Irish Americans, especially those in New York, may be directly descended, like Gates, from Niall of the Nine Hostages, the most prolific warrior in Irish history.
A team of geneticists at Trinity College led by professor Dan Bradley have discovered that as many as 3 million men worldwide may be descendents of the Irish warlord, who was the Irish "High King" at Tara, the ancient center of Ireland from A.D. 379 to A.D. 405.
The story of Niall of the Nine Hostages is already the stuff of legend, which has been passed on to countless Irish schoolchildren over the years.
The supposedly fearless leader battled the English, the Scots, the French and even the Romans, and struck fear into the heart of his enemies. His dynasty lasted for centuries, continuing up until the Elizabethan conquest of Ireland at the end of the 16th century.
Legend has it that it was Niall of the Nine Hostages who, on a raid in Wales, captured a young slave and brought him to Ireland. That slave would later escape, and go to become Ireland's patron saint, St. Patrick.
But one story not told to most Irish elementary schoolchildren was of Niall's prolificacy.
When it came to the bedroom, it seems that Niall of the Nine Hostages was even more fearless and energetic than he was on the battlefield.
This warlord was responsible for the very common Irish surname "O'Neill" -- which means "descendant son of Niall." It is also the name of Irish pubs all over the world.
The researchers also found that as many as one in 12 men in Ireland have the same DNA as the Irish king -- and in Ireland's northwest, that figure rises to one in five.