The history of the O'Neills could fill volumes such has been their influence on Irish history and culture. The O'Neills are most associated with Ulster Province (the red hand of Ulster is taken from their coat of arms) but there were other septs located outside of Ulster who also anglicized their name as O'Neill.
The O'Neills of Thomond were centered on the area around Bunratty and although O'Neill is not often found there today traces of the sept are hidden under the names Nihill and Creagh.
The O'Neills of Carlow were located at Rathvilly with descendants of another sept found in Counties Tipperary and Waterford.
The Ulster septs of O'Neill, of course, are those most associated with the name. Their ancestor was Niall, King of Ireland, who died while battling the Vikings in the year 919 AD. They were the chief family of the Cinel Eoghan, their territory being modern day Tyrone, Derry and Donegal. These septs were originally of a tribe called 'the Ui Neill' who were the descendants of the famous 'Niall of the Nine Hostages' and who were undisputed Kings of Ireland for centuries. They formed two main branches, the Ui Neill of Ulster and the Southern Ui Neill, located in County Meath.
The O'Neills were so powerful that they were targeted by the English for extermination with special zeal in the sixteenth century. Despite the best efforts of the crown under their agent Essex, the O'Neills continued to thrive in Ulster right up until the 'Flight of the Earls' in 1607, and the departure from Ireland forever of Hugh O'Neill (1540-1616), the second Earl of Tyrone. His departure signalled a new era in Irish life and history as the Gaelic way of life was finally subverted by the superior war machine of the English.
There have been many famous bearers of the name including Shane O'Neill (1530-1567), Sir Niall O'Neill (1658-1690) who distinguished himself at the battle of the Boyne. Arthur O'Neil (1737-1816) was a blind harpist of great renown. John O'Neill (1834-1878) was a Fenian leader. Eugene O'Neill (1888-1953), the dramatist, was the son of an American actor, himself an Irish immigrant. Congress Thomas 'Tipp' O'Neill (1912-1994) was a US Speaker of the House of Representatives and an advocate of the Irish cause.
In modern times the name O'Neill is still found throughout the world in considerable numbers, such was the influence of the mighty families of O'Neill.