As a proper noun, the word Deisi (sg. Deis) means "subject peoples" and was the name borne in the historical period by two important Erainn populations, the one in Brega, the other in Munster. The former were known as both the Deisi Breg and the Deisi Temro (the subject peoples of Tara) because they occupied lands just south of the ancient site. In the eighth century, their kingdom was eclipsed by the expanding Southern Ui Neill dynasty of Sil nAedo Slaine, though with the decline of their overlords, they were able to regain their independence in the eleventh. Their revival, however, did not outlast the subsequent Anglo-Norman Invasion.
Larger were the Deisi populations in Munster, which originally formed a single, if discontinuous, conglomerate stretching from the extreme southeast to the north of the province. In the fourth or fifth century, a branch of the Deisi from the Waterford area established a colony in Dyfed (southwest Wales), where they retained power until the tenth century. Later, their Irish counterparts split into two main divisions about the beginning of the eighth century: The Deisi Muman lived in County Waterford and southern Tipperary and the western Deisi in eastern Limerick. In the latter territory, the most important people were the Deis Tuais-cirt, who conquered east County Clare. By the early tenth century, they adopted the name Dal Cais and subsequently became the most powerful kingdom in Ireland, under their ruler Brian Boru (d. 1014).
