O'Brien Dynasty - Background

Background

The O'Briens emerged as chiefs of the Dalcassian race from the south-west of Ireland — a cohesive set of septs, related by blood, all claiming descent in tradition from a common ancestor of Cormac Cas. In the Annals of the Four Masters, the father of Cormac Cas was said to be Oilioll Olum, who was according to tradition King of Munster and King of Leinster in the 3rd century. Such a connection would have meant that the race held kinship with the Eoghanachta who had dominated Munster since the earliest times. While founder mythologies were very common in antiquity and the medieval world, such a connection is generally regarded as fanciful and politically motivated in the context of the rise to prominence of the Dalcassians.

Instead, academic histories generally accept the Dalcassians as being the Déisi Tuisceart, after adopting a new name — first recorded under their newly adopted name under the year 934 in the Annals of Inisfallen. The Déisi, a people whose name means literally vassals, were originally located where today is Waterford, south Tipperary and Limerick; the O'Rahilly's historical model counts them as ethnically Érainn; the sept split into the Déisi Muman who continued to hold territory in Waterford and Tipperary, while the west Déisi controlled areas either side of the River Shannon. During the 8th century, the latter was further divided into the Déisi Deiscirt and the Déisi Tuisceart who would become the Dalcassians. Prehistoric ancestors of the Déisi Tuisceart and Dál gCais may have been a once prominent Érainn people called the Mairtine.

It was during this century that the race annexed to Munster the area today known as Clare and made it their home. Taken from the weakened Uí Fiachrach Aidhne it had previously been part of Connacht but was renamed Thomond (Tuamhain, meaning North Munster). After gaining influence over other tribes in the area such as the Corcu Mruad and Corcu Baiscinn, the Dalcassians were able to crown Cennétig mac Lorcáin as King of Thomond, he died in 951. His son Mathgamain mac Cennétig was to expand their territory further according to the Annals of Ulster; capturing the Rock of Cashel capital of the Eoghanachta, the Dalcassians became Kings of Cashel and Munster over their previous overlords for the first time in history.

Mathgamain along with his younger brother Brian Boru began military campaigns such as the Battle of Sulcoit, against the Norse Vikings of the settlement Limerick, ruled by Ivar. The Dalcassians were successful, plundering spoils of jewels, gold and silver, saddles, finding "soft, youthful, bright girls, booming silk-clad women and active well-formed boys". The males fit for war were executed at Saingel, while the rest were taken as slaves. Through much of his reign Mathgamain was competing with his Eoghanachta rival Máel Muad mac Brain. Mathgamain was only defeated in the end by a piece of treachery; he believed he was attending a friendly meeting, but was betrayed at Donnubán mac Cathail's house, handed over to his enemies and executed in 976. The crown of Munster was briefly back in the hands of the Eoghanachta for two years until Brian Boru had thoroughly avenged his brother, with the defeat and slaying of Máel Muad in the Battle of Belach Lechta.