Donnchad Donn mac Flainn (d. 944), high-king of Ireland, was the son of , and of Gormlaith, daughter of Flann mac Conaing (d. 868), king of all Brega (Knowth dynasty); the epithet Donn may be translated as the Brown-Haired. A member of the Clann Cholmáin, the main branch of the southern Uí Néill based on the crannog of Cró-inis in the south-west corner of Lough Ennell in Westmeath, he became king of Mide in 916 and succeeded to the high-kingship in 919 on the death of the northern Uí Néill high-king, Niall mac Áeda (Niall Glúndub).
According to the Irish annals Donnchad had three wives. Cainnech, who died in 929, was a daughter of Canannán of Tír Conaill (modern Donegal). Órlaith was killed by Donnchad in 941, after what may have been a sexual relationship with his son Óengus; she was a daughter of Cennétig mac Lorcáin of the Dál gCais of Munster. Dublemna died in 943; she was a daughter of Tigernán mac Sellacháin, king of Bréifne (approximately modern counties Leitrim and Cavan). His sons were Conn (d. 944), Óengus (d. 945), and Domnall Donn (d. 952); and his daughters were Flann (d. 940) and Óebfhinn (d. 952).
Donnchad first appears in the historical record in 904 when his father, Flann, opposed him in the town of Kells, beheading many of his associates in the vicinity of the church. With his brother Conchobor he again rebelled against his ageing father in 915. They were brought to heel by Niall Glúndub, then king of Ailech, who was protecting his own future interests, especially since his attempt to oppose Flann in the previous year had failed. On succeeding his father, Donnchad ruthlessly removed opponents, blinding his brother Áed in 919, and killing his brother Domnall in 921 (which was fitting, according to the annalist) at Bruiden Dá-Choca (Breenmore, Westmeath, on the east side of the River Shannon). This was almost certainly because of Domnall's close relationship with the Connachta, as is confirmed by Donnchad's unsuccessful raid across the Shannon in the following year. He killed his nephew, Conchobor's son Máel Ruanaid, in 928.
Donnchad campaigned with his kinsmen against the men of Bréifne in 910; and in 913 alongside Máel Mithig mac Flannacáin (his sister Lígach's husband) of the Knowth dynasty (also his mother's people) against Lorcán, son of Dúnchad (a dissident of the Knowth dynasty), who was in league with the southern Brega dynasty at Lagore and the Leinstermen. Indeed his campaigns in Brega in 939 and 940 were probably against Lorcán's two sons who were finally dispatched by Congalach, Máel Mithig's son, in 942.
After assuming the high-kingship Donnchad inflicted a massive defeat on the vikings in Louth in 920. His ally Muirchertach mac Tigernáin of Bréifne (brother of his wife Dublemna) died from his wounds. Donnchad was opposed by the northern Uí Néill king, Muirchertach mac Néill, in 927, 929, and 938, but on each occasion conflict was avoided, probably because his daughter, Flann, was married to Muirchertach. They raided Leinster and Munster jointly in 938 and 940; and in an independent campaign in 941 (following the death of Flann in 940) Muirchertach plundered Mide, Uí Failge, Osraige, and Déisi and delivered Cellachán of Cashel to Donnchad as hostagethus demonstrating that Donnchad's power was always limited.
Through his marriages Donnchad was connected with future powerful familiesthat to Cainnech of the Cenél Conaill foresaw the rise of Ruaidrí Ua Canannáin, as that to Órlaith recognized the rise of the ancestors of the Uí Briain. Similarly, the marriage to Dublemna acknowledged the rise of the ancestors of the Uí Ruairc. In 939 Donnchad had a shrine made for the Book of Armagh. He died in 944.
Ann. Ulster · W. M. Hennessy, ed. and trans., Chronicum Scotorum: a chronicle of Irish affairs, Rolls Series, 46 (1866) · AFM · S. Mac Airt, ed. and trans., The annals of Inisfallen (1951) · D. Murphy, ed., The annals of Clonmacnoise, trans. C. Mageoghagan (1896); facs. edn (1993) · M. C. Dobbs, ed. and trans., The Ban-shenchus [3 pts], Revue Celtique, 47 (1930), 283339; 48 (1931), 163234; 49 (1932), 43789 · P. Walsh, The Ua Maelechlainn kings of Meath, Irish Ecclesiastical Record, 5th ser., 57 (1941), 16583