Ui Dunlainge was one of the most important population groups and dynasties in early medieval Leinster. Ui Dunlainge considered themselves to be of the Laigin and to be descended from Dunlaing, grandson of Bres-sal Belach, the ancestor also of Ui Chennselaig. Ui Dunlainge occupied fertile land in north Leinster, including the Liffey valley and Kildare plains. Ui Dunlainge initially owed their rise to the decline of earlier Leinster dynasties in the face of the expansion of the southern Ui Neill, and in the seventh century they drove the Ui Garrchon and Ui Enechglaiss across the Wicklow mountains. The first significant king of Ui Dunlainge was Faelan mac Colmain (d. 666 or earlier) who defeated competitors from the rival dynasties of Ui Mail and Ui Chennselaig to attain the kingship of Leinster; he was also allied with the southern Ui Neill.
From the mid-eighth century, Ui Dunlainge monopolized the kingship of Leinster. Three of the sons of Murchad mac Brain (d. 727), Dunchad, Faelan, and Muiredach, reigned in turn after him as kings of Leinster. These kings were progenitors of the most powerful branches of Ui Dunlainge in the following three centuries: Ui Dunchada, Ui Faelain, and Ui Muiredaig. Ui Dunchada were settled between the lower Liffey and the Wicklow mountains, their territory later extending to the outskirts of Dublin. Their center was at Liamain (Lyon’s Hill, Co. Kildare), and several members of the family enjoyed the abbacy of Kildare. Ui Faelain settled to the southwest, with their center at Naas. Further south dwelt Ui Muiredaig, with their base at Maistiu (Mullaghmast, Co. Kildare); they had links with the church of Glendalough.
The kingship of Leinster rotated between these groups, and this pattern of a "circuit among the branches" of an Irish dynasty has been used as one of the models of Irish kingship. For all the apparent neatness, the succession was often accompanied by feud and fratricide. Additionally, though later king-lists call many of these dynasts kings of Leinster, the contemporary evidence of the annals sometimes gives them lesser titles such as "king of Ui Dunlainge," rt Iarthir Liphi ("king of western Liffey"), or even "king of Naas." This may be due to the dominance that the Ui Neill periodically asserted over them, forcing them to submit and give hostages. The southern Leinster kings must often have rejected Ui Dunlainge overlordship, and as time went on the authority of the Ui Dunlainge kingship was eroded.
From the ninth century, domination of Leinster became a main point of contention between the kings of Ui Neill and the kings of Munster. A highly significant development in this period was the establishment of a Viking base at Dublin in 841. Soon the Dublin Vikings controlled a substantial hinterland north and south of the Liffey estuary (Fine Gall) and were a considerable threat to their neighbors. However, internal dissensions among the Dublin Norse facilitated an attack in 902 by Cerball mac Muirecain, Ui Faelain king of Leinster, in alliance with the king of Brega, which drove the Vikings from Dublin for fifteen years. Cerball Mac Muirecain also fought against Cormac mac Cuillenain in 908 and married Gormfhlaith (d. 948), said to have been the latter’s widow.
Uf Dunlainge also had to contend with the kings of Osraige, whose power grew steadily in the late ninth and tenth centuries. Cerball mac Dungaile and his son Diarmait mac Cerbaill, both kings of Osraige, allied with Vikings and attempted to dominate Leinster. The Uf Dunlainge themselves quickly learned the advantages of allying with Vikings; in 956 the Leinstermen and Dublin Vikings killed the king of Tara. In the late tenth century Dublin and its hinterland, though politically independent of Leinster, was seen as an important center of wealth and power, and Irish kings, including those of Uf Dunlainge, attempted to assert control of the settlement.
In the 980s and 990s, Uf Dunlainge lost out as Brian Boru attempted to control Leinster and Dublin as part of his struggles with Mael-Sechnaill II for the high kingship of Ireland. In 997, Mael-Sechnaill gave to him the hostages of Leinster and Dublin he had previously held. The Laigin and Dublin Vikings were no more amenable to the overlordship of Brian than to that of the Uf Neill kings, and they rebelled in 999, to be crushed by Brian. His success in Leinster was in part due to the divisions between Uf Dunlainge and Uf Chennselaig, and between the branches of Uf Dunlainge themselves. In 1003, Brian deposed Donnchad mac Domnaill of Uf Dunchada as king of Leinster and installed Maelmorda mac Murchada of Uf Faelain. Brian was in fact married to Maelmorda’s sister, Gormfhlaith (d. 1030), who had previously been married both to Amlafb Cuaran, king of Dublin, and to Mael-Sechnaill. However, relations deteriorated over the following years, and Maelmorda rebelled against Brian. This culminated in the Battle of Clontarf in which both were killed. The power of Osraige subsequently grew again. Donnchad mac Gilla-Patraic, king of Osraige, intervened in Leinster several times, and in 1036 he took the kingship of the province. A fatal blow had been dealt to declining Uf Dunlainge power, and in 1042 the kingship of Leinster passed to Uf Chennselaig in the person of Diarmait mac Maele-na-mBo.
Uf Dunlainge retained considerable power in their own districts into the twelfth century. Uf Dunchada had suffered territorially at the hands of the Dublin Vikings, although they retained land at Lyon’s Hill and in the area of the Dublin-Wicklow border. Their family name at the time of the Anglo-Norman invasion was Mac Gilla Mo-Cholmoc, and their descendants survived under the new regime as the Fitzdermots. The English invaders forced the Uf Faelain and Uf Muiredaig dynasties, at that time represented by the families of Ua Brain (O’Byrne) and Ua Tuathail (O’Toole) eastward into the less fertile lands of the Wicklow Mountains. However, they were able to survive and even prosper as Gaelic lordships until the end of the Middle Ages.
