UI Chennselaig was one of the most important population groups and dynasties in early medieval Leinster. Uf Chennselaig considered themselves to be of the Laigin and traced their descent to one Enna Cennselach (“Enna the Dominant”), a grandson of Bressal Belach, the ancestor also of Uf Dunlainge. It is probable that Uf Chennselaig originally came from the Barrow valley and moved eastward into central Leinster, where their early royal center was Rath Bilech (Rathvilly, Co. Carlow); later they expanded southward across the Blackstairs Mountains into the fertile plains of County Wexford and won supremacy in the south of the province, so that their rulers were occasionally styled ri Laigin Desgabair (“king of south Leinster”) in the annals.
The first significant Uf Chennselaig king was Brandub mac Echach (d. 605) who defended Leinster against Uf Neill encroachments. Despite this success, Brandub’s branch of the dynasty, Uf Felmeda of Rath Bilech, were unable to compete with Uf Dunlainge of north Leinster, and their power was eclipsed. In the eighth century, members of Sfl Cormaic and Sfl Maeluidir competed for the Uf Chennselaig kingship. Some of them also became kings of Leinster, but after 738 Uf Dunlainge excluded the other Laigin dynasties from the provincial kingship for over three hundred years. Accordingly, Uf Chennselaig set about consolidating their hold on south Leinster: Sfl Cormaic expanded into the territories of Uf Drona (south Co. Carlow) and the area around the church of St. Mullins on the Barrow with which Uf Chennselaig had long-standing associations; Sil Maeluidir had meanwhile taken control of the lower Slaney and the area adjacent to Wexford harbor, in the process isolating earlier Leinster peoples, Uf Bairrche and Fothairt.
In the ninth century, Uf Chennselaig power was centered on the church of Ferna Mor Maedoc (Ferns, Co. Wexford). Uf Chennselaig had close associations with the church, and several members of the dynasty also held ecclesiastical office there, sometimes in combination with the kingship; Cathal mac Dunlainge is titled rex nepotum Cennselaig et secnap Fernann (“king of Uf Chennselaig and prior of Ferns”) in the annals at his death in 819. During this period, Viking incursions in south Leinster led other churches to look increasingly to the protection and patronage of Uf Chennselaig kings, and so the annals report Cairpre mac Cathail fighting with the muinter (“community”) of the church of Tech Munnu (Taghmon, Co. Wexford) against Vikings in 828. Viking activities had an increasing effect on the intradynastic struggles of Uf Chennselaig, particularly after the establishment of Viking settlements at Wexford and Waterford. From the late ninth century, branches of the dynasty including Sfl Cormaic, Sfl nEladaig, and Sfl nOnchon were all contenders for the overall kingship. Sfl nOnchon, originally an obscure group, provided two kings of Uf Chennselaig in the late ninth century, Tadg mac Diarmata (d. 865) and his brother Cairpre (d. 876). Although both these kings were killed feuding with their own relatives, most of Cairpre’s descendants in the tenth century were kings of Ui Chennselaig, and were gradually able to increase their power despite internecine strife and intrusions from outside south Leinster. Diarmait mac Maele-na-mBo of Sfl nOnchon inaugurated a new period of success for the dynasty, firstly by consolidating his hold on Ferns and the rich royal demesne thereabout, and then eliminating rivals for the kingship of Ui Chennselaig. The power of the Ui Dunlainge kings had declined as a result of attacks by Ui Neill, Vikings, and the kings of Osraige, and Diarmait mac Maele-na-mBo was able to take the kingship of all Leinster in 1042. While he was engaged on his campaigns, his son Murchad acted as regent in Leinster, maintained Ui Chennselaig dominance over Ui Dunlainge, conducted border raids against Mide, and focused his attention particularly on the control of Dublin, which was increasingly integrated into the Leinster province. Murchad’s obit in 1070 calls him rt Laigen & Gall (“king of Leinster and the Foreigners”), and he was buried at Dublin. His descendants took the family name of Mac Murchada (Mac Murrough), and though they retained the Leinster kingship they were unable to challenge for the high kingship of Ireland, though Diarmait Mac Murchada played a pivotal role in the politics of the twelfth century.
