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This surname, which is one of the very few to have retained not only the O prefix but also the Gaelic spelling, is among the ten most numerous in Ireland, and one of the most famous and prominent in the history of Ireland. It is among the first ten names in Counties Antrim, Derry and Tyrone.

The O’Neill sept trace their lineage to Eogan, son of the 5th century High King of Ireland, Niall of the Nine Hostages, who ruled from the Hill of Tara, County Meath. Eogan and his brother Conall Gulban conquered northwest Ireland, ca.425 AD, capturing the great hill-fort of Grianan of Ailech in County Donegal.

Where Does the Last Name O’Neill Come From?

Eogan, styled ‘King of Ailech’, established his own kingdom in the peninsula in County Donegal still called after him Inishowen (Innis Eoghain or Eogan’s Isle). His descendants, known as the Cenel Eoghain (the race of Owen), became the principal branch of the Northern Ui Neill (descendants of Niall of the Nine Hostages). The Cenel Eoghain in the next five centuries expanded to the east and south from their focal point in Inishowen.

Ireland was one of the first countries to adopt a system of hereditary surnames which developed from a more ancient system of clan or sept names. The surname was formed by prefixing either Mac (son of) or O (grandson or descendant of) to the ancestor’s name.

The first to take O’Neill as their surname was Domhnall who took the name of his grandfather, Niall, Black Knee, High King of Ireland who was killed in a battle with the Vikings in 919 AD. The O’Neills were thus in Gaelic O Neill i.e. grandson of Niall.  In the middle years of the 11th century the O’Neills moved their capital from Ailech to Tullaghoge (near Cookstown, County Tyrone).

After a decisive battle in 1241 at Caimeirge (which scholars believe was near Maghera in County Derry) the O’Neills ousted the McLaughlins as the senior branch of the Northern Ui Neill. Brian O’Neill was now ‘installed in the lordship of the Kinel-Owen’, his territory being Tir Eoghain (Tir Owen or Tyrone, the land of Owen) which extended over the present counties of Tyrone and Derry.  

The senior branch of this sept, the O’Neills of Tyrone, were frequently High Kings of Ireland, and in the 16th and 17th centuries they were the leaders of Gaelic resistance to English attempts to pacify Ireland. A junior branch established themselves in County Antrim in the 14th century and from their seat at Shane’s Castle became known as the Clannaboy or Clandeboy O’Neills.

O’Neills Who Have Marked their Surname in History

The escape of the most prominent Gaelic Lords of Ulster, including Hugh O’Neill who was styled the ‘Prince of Ireland’, in ‘the Flight of the Earls’ in 1607 from Lough Swilly marked the end of Gaelic power and paved the way for the 17th century Plantation of Ulster with English and Scottish settlers.

In the 17th and 18th centuries many descendants of the old Gaelic order in Ireland emigrated, as the so-called Wild Geese, to Europe, and, in particular, to Spain and France. The European branches of the O’Neills distinguished themselves in the military of France, Spain and Portugal.

Add your O'Neill Ancestors

This surname history has been researched and written by Brian Mitchell. Brian has been involved in local, family and emigration research in Derry and North West Ireland since 1982. The database whose construction he supervised, containing one million records (dating from 1628 to 1930) extracted from the major civil and church registers of County Derry, can now be accessed at Brian can be contacted by email at

Parish(es) Antrim Derry Donegal Tyrone
Category (ies) Surname