https://www.irishamerica.com/2019/03/roots-the-ferocious-and-fascinating-oneills/

The Uí Néill (meaning “descendants of Niall”) were a powerful and influential collection of Irish dynasties that dominated the northern half of Ireland from the 6th to the 17th century. Claiming descent from the semi-mythic 5th-century High King Niall of the Nine Hostages, they shaped much of Ireland’s early laws, legends, and political structure. 

The Uí Néill dynasty was historically divided into two primary branches, each descended from different sons of Niall: 

1. The Northern Uí Néill

Ruling across modern-day Donegal, Derry, and Tyrone, the Northern branches held vast influence and successfully resisted Anglo-Norman and later Tudor encroachments. 

• Cenél nEógain: Descended from Niall’s son Eógan, this sept became the preeminent Gaelic lords of Ulster, styling themselves as The O’Neill (Kings of Tyrone). Their power base and inauguration site was located at Tullaghoge in County Tyrone. 
• Cenél Conaill: Descended from Niall’s son Conall Gulban, these were the ancestors of the powerful O’Donnell clan, who ruled the neighboring Kingdom of Tír Chonaill. 

2. The Southern Uí Néill

Centered in the midlands of Ireland (predominantly the Kingdom of Mide, encompassing modern Meath and Westmeath), the Southern Uí Néill initially held the paramount title of King of Tara. 

• Key Clans: This branch included the Cenél Fiachach, Síl nÁedo Sláine, and Clann Cholmáin.
• History: They were deeply involved in the High Kingship, but their power began to fracture following the death of High King Máel Sechnaill mac Domnaill in 1022, after which their influence waned. 

Cultural Impact and the “Flight of the Earls”

For roughly 300 years, the Northern and Southern Uí Néill dynasties alternated the supreme High Kingship of Ireland between their houses. []

The power of the premier dynasty, the O’Neills, finally crumbled at the beginning of the 17th century. After resisting the Tudor conquest for decades, Hugh O’Neill, the 2nd Earl of Tyrone, was defeated and forced into exile in 1607—a pivotal historical event known as the Flight of the Earls. Their lands subsequently fell to the Crown, triggering the Plantation of Ulster. 

Today, the O’Neill surname is one of the most distinguished in Ireland. Geneticists have even suggested that the Uí Néill dynasty may have left a massive genetic footprint, with potentially millions of people worldwide sharing the Y-chromosome signature of Niall of the Nine Hostages.