The O'Neills can trace their family history back to A.D. 360, a rare feat among the families of Europe. They are descended from the royal family of Tara, who were kings of Ulster and monarchs of all Ireland from the 5th to the early 17th centuries. The name comes from Nial Gl.n Dubh, or Niall of the Black Knee, who was a King of Ireland from 890 until he was killed in 919. His grandson Domhnall adopted the surname Neill, which means champion. In addition to the O'Neills of Ulster, where the family is most numerous, there are septs in Thomond (counties Clare and Limerick), Decies (Co. Waterford), and Co. Carlow.

The name is distinguished both within and beyond Ireland. Hugh O'Neill (1550-1616), the Second Earl of Tyrone, was defeated by the English at the battle of Kinsale in 1601. He was the last great leader of Gaelic Ireland. But in 1646, Owen Roe O'Neill defeated an English and Scottish army at Benburb, Co. Tyrone. Much material on the O'Neills can be found in the O'Neill historical centre there.