The Dal gCais were the great clan of Thomond, the DALCASSIANS North Munster, an area more especially associated with County Clare (excluding the Burren and Corcomroe on the northwest corner) and adjacent parts of Tipperary and Limerick. They were the axe-wielding footsoldiers who formed the core of the army that defeated the Vikings in 1014, one of the most significant dates in Gaelic history. The chief families of this tribe were above all the O’Briens, but also the MacConsidines, MacDonnells, MacLysaghts, MacMahons, O’Ahernes, O’Kennedys, O’Shanahans, O’Duracks, MacGraths, O’Fogartys, O’Galvins, O’Gradys, O’Hanrahans, O’Hickeys, O’Mearas, O’Molonys, O’Moroneys, O’Haruzgans, O’Lonergans, Creaghs, O’Quins, MacNamaras, Maclnerneys, O’Deas and O’Griffeys.
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The Ó mBloid and Ó gCaisin are given as Dalcassian septs, descended from the sons of Cas, 7th in descent from Ailell Aulum (or Oilill Olum), and represented by the septs of O'Brien, MacNamara (Clann Chuiléin), and many others in the region. The O'Briens and MacNamaras were dominant lords in this region for many centuries.
The Cenél Fearmaic are given as either Úi Fidgeinti or Dalcassian in origin, depending on which genealogy followed, and were represented by the O'Deas, O'Quins, O'Heffernans, et al. Origins of the Uí Cormaic are given as Úi Fidgeinti with their representatives the O'Hehirs, et al. The Corca Baiscinn are given in descent from a son of Conaire Mor (c. A.D. 165) of Dalriada, and related to the Corco Duibne of Munster and the Dal Riata of Ulster, with their later representatives cited as O'Donnell and MacMahon of Munster, among others.
The Uí Breacain are given as a Dalcassian sept in early genealogies, with their representatives including . The main septs of the Corco Mruadh, in descent from the Clanna Rory of Ulster, included O'Connor of Corcomroe, O'Loughlin of the Burren, O'Garbh, among others. The Tradaree (Tradraige) area included, among others, the Ó Neills of Cenél Dealbhaoith. O'Donovan's Tribes of Uí Fiachrach cites the tribes of the Tradraighe as remnants of the original Firbolg tribes of Connacht. Only the Vikings, fierce seafarers from Norway, made permanent inroads in Ireland. They came in the centuries after AD 795, founding fortified trading colonies at Dublin, Cork, Limerick, Waterford and Wexford.
CS1026, An army was led by Brian's son and he took the hostages of Mide and Brega and the foreigners and the Laigin and the Osraige.
The O’Briens (O Briain) were the chief family of the Dal gCais, otherwise known as the Dalcassians, and the heads of the family were kings of Thomond. The O’Briens derive their descent from Brian Boru, High King of Ireland, who was slain at Clontarf in 1014, at the moment of his final decisive victory over the united Viking army. His individual career is remarkable in the history of the Gael, and is so distinguished and outstanding that it cannot be mitigated by faint praise, for among other things he brought a degree of unity and common purpose to the Gael. He came out of virtual obscurity to bring Ireland out of its perpetual chaos just enough to guarantee its existence into the future. Such was his greatness that neither his nobility nor the quality of the seed that produced him can ever be called into question (as a tree bears fruit in kind, so a man’s parentage and ancestry were of central social and political importance to the Gael, hence the family names in "O" and "Mac". For those interested, the book Lion of Ireland by Morgan Llywelyn gives his life story with remarkable accuracy and human interest.
It was Brian who raised his clan, the Uí Toirdealbhaigh, to preeminence among the Dalcassians, although there is evidence that the Uí Toirdealbhaigh were originally from Connacht. He laid the foundation for his progeny’s future greatness, just as the guerrilla tactics of the Uí Tordealbhaigh, under Brian’s leadership, laid the foundation of Brian’s later reputation and success as a Viking-stopper. The O’Briens became not only the ruling family in Thomond, but some of them were over-kings of Munster and some High-Kings of Ireland as well. Their own possessions included the whole of County Clare, and large parts of Tipperary, Limerick, and Waterford as well.
The O’Briens divided into several branches, the most important of which were the O’Briens of Ara, in northwest Tipperary, the chief of which was known as Mac I Bhriain Ara; those of Coonagh in the east of County Limerick; those of Pobelbrien, now the barony of that name in County Limerick (their chief stronghold was Carrigogonnell, on the Shannon); those of Aherlow, in Tipperary, and finally those of Cumaragh, in County Waterford, who had extensive possessions along the Cummeragh Mountains, that is, the valley between Dungarvan and the Suir. Other families of the Uí Toirdealbhaigh, whose original territory was in the east of County Clare, include the MacConsidines, MacLysaghts, MacDonnells and MacMahons.