Ó Néill- anglicised O' Neill, earlier O' Neal, Neale etc. Woulfe gives the meaning of Niall as 'Champion' ('Sloinnte Gaedheal is Gall', 1923).
Apart from the great Ulster O' Neills, there are several septs of O'Neill, quite disparate, across Ireland:
The O' Neills of Thomond, ancient chiefs in Bunratty, Co Clare; the O' Neills of Co Carlow, a sept located in Rathvilly Barony; the O' Neills of the Decies in Co Waterford, and the name is prevalent enough in the adjacent counties of Cork and Tipperary. None of these septs are related to the Ulster O' Neills, but are interesting in their own right.
The Ulster O' Neills have been one of the most prestigious of Irish families (the red hand of Ulster is taken from their arms) and descend from Niall Glun Dubh 'black knee', High King of Ireland, killed in 919 by the Norsemen. The first to use the name was Domhnall Ó Néill, the latter's grandson. The O' Neills are just one family of the tribal grouping Uí Néill, who all claim descent from the legendary Niall of the Nine Hostages. MacLysaght writes that the surname goes back to the 10th century king, rather than the legendary hostage taker ('Irish Families', 1982).
The O' Neills were the leading family of the tribal group the Cinel Eoghain, from Eoghain the son of Niall 'Noighiallach' (of the nine hostages). Their original territory comprised the modern counties of Tyrone, Donegal and Derry. A branch were established in Antrim; these were dubbed Clann Aodha Bhuidhe, the family, or tribe, of Hugh Boy, who was killed in 1283.They were de facto kings of Ulster; it was diplomacy that prompted Henry V111 to make Conn O' Neill the Earl of Tyrone, an attempt to curb their power. To further dismantle their Gaelic identity, Queen Elizabeth had their Tullahogue inauguration stone broken up.
The Elizabethan English invaders persecuted them with fire and the sword, but they led the Gaelic resistance, and enjoyed a resurgence in the 17th century. Hugh O' Neill [1550-1616] second Earl of Tyrone, was Gaelic Ireland's great leader; the defeat of his army at Kinsale in 1601 was a terrible blow. It was in this century that there appeared two of the finest war leaders Ireland, even Europe, has ever seen: Eoghan Ruadh, angl Owen Roe, who spearheaded the Confederate Gaelic army after 1641; and Aodh Dubh, angl Hugh Duff, who inflicted the heaviest losses Cromwell's New Model Army received in Ireland, at Clonmel in 1650.
In Petty's mid 17th century 'Census' we find a few O' Neills in Donegal, in the form McNellus, a few O Neale in Co Down, more in Co Derry, O Neal, McNeale; and many in Co Antrim, O Neill, McNeill; also many in Co Armagh, O Neile, O Neale etc.
The returns for Co Tyrone are missing.
In the Petty 'Census' there are a good number of O Neall and O Nihill in the southern baronies of Bunratty and Tulla in Co Clare; these are a distinct sept from the above.
In Griffith's 1850s 'Valuation' the top 5 counties for O' Neill households were: Tyrone 461, Antrim 305 +130 Belfast, Armagh 127, and Clare 125. The last would be a different sept, see above.
Most O' Neill births at the end of the nineteenth century occurred in counties Dublin [recte the city] Antrim, Cork [a different sept] and Tyrone.
Jonjo O' Neill, Champion National Hunt jockey, incredible record breaker, and trainer, was born in Co Cork in 1952.