The Uí Neill were the great royal tribal dynasty of the North Gaels. Having separated from the royal kinsmen, the Connachta, shortly alter the career of their illustrious fifth century ancestor Niall of the Nine Hostages, they set out from a base in Sligo and soon (by the beginning of the sixth century) monopolized the Sacral High-Kingship of Tara, which for hundreds of years they alternated between their own two illustrious branches, the Northern Uí Neill and the Southern Uí Neill. The Northern Uí Neill divided into three great clans, the Cenél Eoghain, Cenél Conaill and Cenél Cairbre. Ancient Dal Riada

The Cenél Eoghan were the Royal Clan of the North Gaels, associated with the High-Kingship of Tara, though in the early period they alternated the overkingship of the north with their Cenél Conaill cousins, by the end of the eighth century they had monopolised the overkingship of Ulster and with it the northern representation in the High-Kingship, aided by the fact that they had, with their centrally dominant fortress of Aliech in northeast Donegal, the strategic advantage, together with the energy and will to exploit it. Their original patrimony included the modern baronies of Raphoe and lnishowen in Donegal, but from their center at the great fort of Ailech in Inishowen, they soon spread throughout Derry and much of Tyrone as well (Tyrone, Gealic "Tir Eoghain", the land of Eoghain, is named for them).

The O’Boylans (O Baoigheallain) were, after the Anglo-Norman invasion, lords of all Oriel, a widespread territory stretching from northwest Fermanagh to Oriel and Louth. The O’Boylans were, after the Anglo-Norman invasion, lords of all Oriel, a widespread territory stretching from Fermanagh to Louth. Until the mid-thirteenth century the leading family of the Cenél Eoghain was MacLoughlin (Mac Lochlainn) of lnishowen; in 1241 they lost a decisive battle to their kinsmen the Ó Neills, and afterwards they declined in power, though a branch became established in County Leitrim under the O’Rourkes. East Breifne was often called Breifne O'Reilly, from its princes of chiefs having from remote ages borne that name: they were tributary to the O'Nial of Tirowen long before the arrival of the English. The Uí Tuirtre and the O’Hegartys (O hEighceartaigh) of the Cenél Eoghain were chiefs in the present barony of Loughinsholin in the south of County Derry, and by about the beginning of the seventeenth century some of them settled in the baronies of Barrymore and Carbery West in County Cork. The family was numerous in the Irish Brigades of France, and several O’Hegartys were, during the eighteenth century, particularly distinguished in that service.

The Gaelic Cenél Eoghain: The O’Beolairts (O Beollain) or Gillanders (Giolla Aindreas) of the Gaelic Cenél Eoghain were co-arbs (hereditary abbots) of St. Maelrubha at Applecross in Ross-shire. They were a powerful princely family, and became earls of Ross in the early thirteenth century. Towards the end of the fourteenth century they inherited the chiefship of the Clann Aindreas, or Clann Giolla Aindreas (Clan Gillanders), a native Pictish tribe related to the MacKenzies and Mathesons of Cromarty and among whom they had long been ecclesiastical and secular leaders. At about the same time they were artificially dispossessed of the Earldom of Ross by the King of Scots, and afterwards the family adopted as a surname what had for some time been the descriptive epithet of (de) Ross.

They are also known by the patronymic of MacAndrew (Mac Gille Aindreas) from the clan name, while the original family name of O Beollain survives as MacBeolain, following Scottish prefix usage. A branch of the O’Beolains became hereditary abbots (erenaghs) of the Columban church at Drumcliffe in Sligo, and were famous for their hospitality. Some of the MacAndrews settled in the Clan Chattan country , and sought the protection of the MacKintosh about 1400. The MacBeolains occupied Glenshiel and the south side of Loch Duich as far as Kylerhea. Fearn abbey was a Premonstratensian establishment of the order of St Augustine by Fearcher MacTaggart (Mac an tSagairt—"the son of the priest") of Applecross, created Earl of Ross in 1234.

1, 2,