The Ulaid (old Irish, pron. /'ʊləðʲ/; modern Irish Ulaidh, pron. /'ʊləɣʲ/) were a people of early Ireland who gave their name to the modern province of Ulster (modern Irish Cuige Uladh, pron. /'kuːiɡʲə 'ʊləɣ/, "province of of the Ulaid", in Irish; the English word "Ulster" derives from Irish Ulaid and Old Norse staðr, "place, territory").
Ulaid is a plural noun, indicating an ethnonym rather than a geographic term. The Ulaid are probably mentioned in Ptolemy's 2nd century Geography, as the Ούολουντοι (Uolunti), probably a corruption of Ούλουτοι (Uluti). The name is probably derived from ul, "beard".
The use of the word cuige, earlier cóiced, literally "fifth", to mean "province", implies the existence at some point in prehistory of a pentarchy, whose five members were the Ulaid, (Ulster) the Connachta (Connacht), the Laigin (Leinster), Mumu (Munster), and probably Mide (Meath), a central province whose name survives in Counties Meath and Westmeath, although the original Mide was more extensive than those counties.
However, that pentarchy no longer existed by the 5th century, when documentary history in Ireland begins. The Ulaid still held significant territory, primarily in Counties Antrim, Down and Louth, although the boundaries of their territories were fluid. Their primary ruling dynasty was the Dál Fiatach, based in Downpatrick, County Down. The name Ulaid developed an additional geographical sense, so that the term rí Ulad, "king of the Ulaid", could refer to the king of the Dál Fiatach, or to the over-king of the north-east, many of whom came from dynasties of the Cruithne such as the Dál nAraide and the Uí Echach Cobo (the Dál nAraide even claimed in its genealogies to be na fir Ulaid, "the true Ulaid"). The rest of the ancient fifth fell under the control of the Airgialla and the northern dynasties of the Uí Néill. The Ulaid as a distinct people, as represented by the Dál Fiatach, survived until their conquest in 1177 by the Anglo-Norman knight John de Courcy.
The Ulaid feature in Irish legends and historical traditions of prehistoric times, most notably in the group of sagas known as the Ulster Cycle. These stories are set during the reign of the Ulaid king Conchobar mac Nessa at Emain Macha (Navan Fort, near Armagh) and tell of his conflicts with the Connachta, led by queen Medb and her husband Ailill mac Máta. The chief hero is Conchobar's nephew Cú Chulainn, and the central story is the proto-epic Táin Bó Cúailnge, "The Cattle Raid of Cooley".
In some stories Conchobar's birth and death are synchronised with those of Christ, which creates an apparent anachronism in the presence of the Connachta. The historical Connachta were a group of dynasties who traced their descent to the legendary king Conn Cétchathach, whose reign is traditionally dated to the 2nd century. However, the chronology of early Irish historical tradition is inconsistent and highly artificial. One early saga makes Fergus mac Léti, one of Conchobar's predecessors as king of the Ulaid, a contemporary of Conn, and Tírechán's 7th century memoir of Saint Patrick says that Cairpre Nia Fer, Conchobar's son-in-law in the sagas, died only 100 years before the saint, i.e. in the 4th century.
Kenneth Jackson, based on his estimates on the survival of oral tradition, also suggested that the Ulster Cycle originated in the 4th century. Other scholars, following T. F. O'Rahilly, propose that the sagas of the Ulster Cycle derive from the wars between the Ulaid and the midland dynasties of the Connachta and the nascent Uí Néill in the 4th and 5th centuries, at the end of which the Ulaid lost much of their territory, and their capital, to the new kingdoms of the Airgialla. Traditional history credits this to the Three Collas, three great great great grandsons of Conn, and dates their defeat of the Ulaid king Fergus Fogae and the destruction of Emain Macha to AD 331. O'Rahilly and his followers believe the Collas are literary doublets of the sons of Niall Noígiallach, eponymous founder of the Uí Néill, who they propose were the true conquerors of Emain in the 5th century.