In 1177 the Anglo-Norman adventurer John de Courcy conquered the kingdom of the Ulaid and established his own lordship based upon its historical center in and around Downpatrick. This action brought to an end one of the most enduring polities in Irish history. In his "Life of Patrick," the late-seventh-century writer Muirchu described the territory of the Ulothi as lying between the Boyne and the Lagan. The same territory seems to be ascribed to the Uoluntii by the Greco-Roman geographer Ptolemy in the second century a.d. In the Ulster Cycle, surviving in literary texts from the eighth century onward, the territory of the Ulaid was said, in pre-Patrician times, to have extended over the whole of Ireland north of the Boyne, but the coincidence of Ptolemy’s and Muirchu’s location of the tribe makes this seem unlikely. It should also be noted that Ptolemy’s Isamnion (O. I. Emain) is a coastal promontory in County Down and not the site near Armagh city with which the medieval authors identified it.
In medieval times, the Ulaid were dominated by the Dal Fiatach dynasty with their royal center at Dun Lethglaise (Downpatrick), who extended their sway over most of modern Down and Louth and parts of Armagh. The tradition of a greater sway than this may have some basis in fact even if it was not as extensive as legends suggest. The Ulaid were counted, along with the Laigin and the Feni, as one of the three major peoples of Ireland, and their claims to hegemony in the northeast were only gradually eroded by the Ui Neill. The Dal Fiatach seem to have been finally marginalized after the battle of Leth Cam in 827 when they seem to have been attempting to detach the Airgialla from Ui Neill overlordship. Indeed, the aspirations leading up to this battle may have inspired the Ulster Cycle vision of a greater Ulaid. Earlier Ulidian kings, such as Baetan mac Cairill (c. 581) and his nephew Fiachnae mac Demmain (626), had been able to operate as major players on the Irish scene.
By the late ninth century, the Dal Fiatach had fallen on hard times and were forced to accept the overlord-ship of their northern neighbors, the Dal nAraide of Moylinny (Antrim). The propaganda produced to legitimize the dominance of this Cruthni dynasty over the Ulaid has obscured the original distinctiveness of the two peoples. Dal Fiatach fortunes were restored by Eochaid mac Ardgail in 972, but inevitably this led to renewed conflict with the Ui Neill culminating in Eochaid’s death in battle in 1004. The period of Dal nAraide dominance coincided broadly with the presence of at least one Viking base on Strangford Loch, which must have discomfited the Ulaid, but in the long run no enduring Hiberno-Norse settlements were established in their territory. The Ulaid even developed their own fleet, which was active in the Irish Sea and even along the British coast.
The dynasty had a final revival under Donn Sleibe macEchdacha (sl. 1091) and his descendants, and developed close relations with the kingdom of the Isles. In the course of the twelfth century Mac Lochlainn kings were twice able to divide the Dal Fiatach into four tigernae ("lordships"), and Mourne was lost to Donnchad Ua Cerbaill, king of the Airgialla, who negotiated the restoration of Ulidian kingship with Muirchertach Mac Lochlainn. After de Courcy’s conquest, the MacDuinnshleibe retained the title "king of Irish Ulster," but they had lost any real independence.
