Donnchad (d. 1039), son of Gilla-Pátraic, king of Osraige and overking of Leinster, belonged to the dynasty of Dál mBirn, which was traced to a proto-historic ancestor, Lóegaire Bern Buadach. Donnchad's father, Gilla-Pátraic son of Donnchad, died in 996 as king of Osraige. While the identity of his mother is unknown, his father was for a time married to Dinim, daughter of Cenn-fáelad king of Múscraige, through which union Donnchad had a brother or half-brother named Domnall. Other siblings included a brother Tadc and a sister Aífe, who was wife in turn to Donnchad Máel na mBó (qv) and to Dúnlaing (qv) son of Tuathal (qv), and mother of the former's sons Diarmait (qv) and Domnall Remur (qv), and of the latter's son Echdonn and daughter Aíbenn. Donnchad himself is said to have married Doirenn, daughter of Dersaid the royal jester of the Leinstermen, but this may merely be propaganda on the part of the Leinster dynasties on whose behalf the ‘Banshenchas’ (lore of women) was compiled. In any event, Donnchad had at least two sons, Domnall and Gilla-Pátraic.
Donnchad first came to notice in 1003 when he slew his first cousin and predecessor in the kingship, Cellach son of Diarmait. Subsequently, he spent several years consolidating his position in the Barrow valley, emerging as a force in high politics only after 1014, when the powerful presence of Brian Bórama (qv), Dál Cais claimant to the high-kingship of Ireland, had been removed from the scene. In the years that followed, Donnchad struggled to free his kingdom from the lordship of Dál Cais, eventually achieving success. As his political strength increased, he asserted supremacy over the Hiberno-Norse kingdom of Waterford and over Leinster – ultimately attaining overkingship of that province.
It appears that the political agreement with the north Leinster dynasty of Uí Muiredaig, reflected in the marriage of Donnchad's sister, ended with the death of Dúnlaing in 1014, if not earlier. The record shows that Donnchad struck against Uí Muiredaig in 1016 and slew their king, Donn-cuan son of Dúnlaing, at the ecclesiastical centre of Lethglenn (Oldleighlin, Co. Carlow). Thereafter, Donnchad maintained a vendetta against his erstwhile allies. He may well have been behind the killing of a brother of Donn-cuan in 1019; certainly he raided Uí Muiredaig territory in 1026, prompting that dynasty to attack Tullamaine by way of revenge. Meanwhile, Donnchad had turned his attentions southwards in an effort to subdue the neighbouring Hiberno-Scandinavian kingdom of Waterford; in 1022, he slew its king, Sitriuc.
By the mid 1020s Donnchad's expansionist moves had attracted the notice of the Dál Cais overking of Munster, Donnchad (qv) (d. 1064) son of Brian, who undertook a major hosting in the spring of 1026 and took hostages from every realm in southern Ireland, including Osraige. That Easter, Donnchad son of Gilla-Pátraic made a formal submission at the Dál Cais caput of Cenn Corad (Kincora). It seems, however, that once back in his own kingdom he reneged on the terms of submission; in 1027, Dál Cais forces again hosted to Osraige, but this time suffered heavy losses. A leading role in the resistance was played by a grandson of Gilla-Pátraic, probably a son of Donnchad. The same year, Donnchad moved to forestall opposition within his own family by blinding his brother Tadc. By 1031 he was sufficiently confident to take the initiative against the Dál Cais overking. Leading an incursion into the heartland of Munster, he burned the fortress of Dún na Sciath and killed its steward. In retaliation, Donnchad son of Brian raided into Osraige and captured much booty, but once more suffered heavy losses.
In 1034 the Munstermen again raided Osraige, but by this time Donnchad had directed his attention towards Leinster. The previous year, it appears, he celebrated the Óenach Carmain (Fair of Carman, east of Kilcullen, Co. Kildare) – which amounted to a claim on the overkingship of Leinster. As Uí Muiredaig continued to resist his ambitions, in 1036 Donnchad captured its leading dynast Dúnchad (or Donnchad) son of Dúnlaing and had him blinded, as a result of which he died. He also captured and blinded Ruaidrí son of Tadc, a nephew of Máel-mórda (qv) son of Lorcán, the tánaise (heir-apparent) of Uí Chennselaig. By this time Donnchad of Osraige was recognised as overking of Leinster; the regnal lists accord him a reign of three years in the provincial kingship. In 1039 he brought Leinster and Osraige forces on an incursion into Meath and burned the area from Cnogba (Knowth) to Drochat Átha (Drogheda).
This was Donnchad's last recorded act; he died on that hosting. It appears that Donnchad's son Domnall was slain the following year by allies of Uí Muiredaig. His other son, Gilla-Pátraic, who succeeded him in the kingship, fought for regional supremacy with the Uí Muiredaig king Murchad (qv) son of Dúnlaing. Gilla-Pátraic reigned as king of Osraige till 1055. From him descended the later kings of Osraige, who took the surname Mac Gilla-Phátraic (Gilpatrick, Fitzpatrick).