Dal Cais was the name of the Munster people based in eastern County Clare that rose to prominence in the latter half of the tenth century and produced a number of powerful kings, including Brian Boru. Although they claimed kinship with the Eoganachta, who had dominated the province since the dawn of history, the Dal Cais actually belonged to the larger population of Munster Deisi, who were ethnically Erainn. This Deisi population originally formed a loose conglomerate stretching from southern Waterford into Limerick, but by the eighth century, they had divided into two separate groups—the Deisi Muman of Waterford and southern Tipperary, and the western Deisi of Limerick. In the latter territory were the Deis Deiscirt, who were eventually eclipsed by their neighbors, and the Deis Tuais-cirt, who later changed their name to Dal Cais. Although legend has it that they conquered their lands in eastern County Clare from Connacht in the fifth century, historical sources suggest that they did not actually gain possession of this territory until the early eighth.
It is not known exactly when the Deis Tuaiscirt adopted the name Dal Cais, but it is first used of them in the Annals of Inisfallen under the year 934, in an entry recording the obit of their king, Rebachan mac Mothlai. His death is also important for marking a major transition in internal Dal Cais politics. For some time prior, the kingship had been controlled by Rebachan’s sept, the Clann Oengusso, but at his death, the office was seized for the first time by their rivals, the Uf Thairdelbaig. Under this new leadership, the Dal Cais began a program of expansion that would soon make them one of the most powerful kingdoms in Ireland.
This expansion began in earnest with the rule of Cennetig mac Lorcain, who had extended his sway over much of north Munster by the time of his death in 951. Building on his father’s success, Mathgamain mac Cennetig extended Dal Cais rule even further by seizing the kingship of Cashel and thus putting an end to centuries of Eoganacht rule in Munster. More importantly, though, Mathgamain also gained control of the Norse settlements of Waterford and Limerick, the resources of which were needed to sustain Dal Cais expansion. In 976, Mathgamain was succeeded by his brother, Brian Boru, who went on to become the most powerful king in Ireland. However, the deaths of Brian and a number of his important kinsmen at the Battle of Clontarf in 1014 crippled Dal Cais. By the time they reemerged in the late eleventh century, the ruling family of Uf Thairdelbaig had adopted the surname Ua Briain (O’Brien), and they enjoyed a considerable revival under Tairrdelbach (d. 1086) and, later, Muirchertach Ua Briain. But with the latter’s death in 1119, the days of the great Dal Cais kings came to an end. After the Anglo-Norman Invasion, the O’Briens were essentially restricted to their lands in Thomond, where they retained some power throughout the later Middle Ages.
