Donnchad Donn (‘the brown-haired’) (d. 944), son of Flann Sinna and king of Tara, belonged to the Clann Cholmáin dynasty of the Southern Uí Néill. His father Flann Sinna (qv) held the kingship of Tara till his death in 916; his mother was Gormlaith, daughter of Flann son of Conaing, Síl nÁedo Sláine king of Brega. He had six brothers or half-brothers, the most prominent of whom were Máel-ruanaid (slain 901 by the Luigne), Conchobar (slain 919), and Domnall (slain 921). He also had two sisters, Gormlaith (qv) (d. 948) and Lígach, who married Máel-mithig, king of Brega, and was the mother of his son Congalach Cnogba (qv). In the course of his life, Donnchad had four wives, his selection of marriage-partners illustrating his priorities in regard to political alliances. First, he married Caíndech (d. 929), daughter of Canannán of Cenél Conaill, and then Derbail (d. 931) daughter of Máel-Finniae of Síl nÁedo Sláine. His later wives were Dublemna (d. 943), daughter of Tigernán of Uí Bríuin Bréifne, and Órlaith, a daughter of Cennétig (qv) (d. 951), of the emerging dynasty of Dál Cais, eldest sister of Brian Bóráma (qv). The sequence of the latter two is not certain; nor is it clear which of his wives mothered his sons Conn, Óengus, and Domnall or his daughter Lann.
Within his father's own lifetime, Donnchad had begun his quest for power and, even by the standards of early Irish dynastic politics, he displayed a marked ruthlessness in securing and in maintaining his claims to kingship. He first came to notice in 904, when he revolted against his father so that the latter was obliged to storm Kells, which resulted in heavy casualties. He again rebelled (915) with the support of his brother Conchobar, and harried Meath, so that the king of Ailech, Niall Glúndub (qv), had to intervene to uphold the position of the elderly Flann Sinna. Meanwhile in 913, Donnchad helped his brother-in-law Máel-mithig, of the Uí Chonaing lineage of Síl nÁedo Sláine, to defeat his Clann Chernaig rivals. When Donnchad's brother Conchobar, having succeeded their father, was slain at Islandbridge by the Dublin Norse along with Niall Glúndub (919), the Clann Cholmáin kingship of Mide passed briefly to his brother Domnall. The latter, who was clearly ineffective, was soon displaced, however, by the ambitious Donnchad. Taking full advantage of the political vacuum that followed the battle of Islandbridge, Donnchad advanced his claims to the kingship of Tara. He held this dignity for twenty-five years according to the regnal lists; the king-list known as ‘Baile in Scáil’ may well have been compiled in his reign. Determined not to tolerate any rivals, he immediately blinded Áed grandson of Máel-Sechnaill (qv), either a half-brother or cousin. Two years later, he slew his brother Domnall; in 928 he slew his nephew Máel-ruanaid son of Conchobar; and in his later years he had his young wife, Órlaith daughter of Cennétig, put to death for alleged infidelity.
In the course of his reign, Donnchad faced military threats from the Dublin Norse and from the Northern Uí Néill; ultimately, he succeeded in playing one off against the other. In 920 he defeated the Norsemen, inflicting considerable losses. In 928 he was engaged in a standoff at Tailtiu with the king of Ailech, Muirchertach (qv) son of Niall Glúndub. A year later, he led an expedition to Liathdruim (Leitrim) against Muirchertach, and in 936 he attacked Dublin with considerable success. After another standoff in 938, it is reported that Donnchad made peace with Muirchertach. It may have been at this stage that the latter married Donnchad's daughter Lann; she is styled ‘queen of Ailech’ in her obit at 940. Certainly an alliance was concluded between the former rivals; together they ravaged the overkingdom of the Dublin Norse, which at that time included much of northern Leinster, from Áth Truisten (near Castledermot) to Dublin. Having paused to deal with opposition of a more local nature in Brega (939, 940), Donnchad again joined forces with his son-in-law Muirchertach to make incursions into Leinster and Munster in which hostages were obtained. In 941 Donnchad secured the Éoganacht king Cellachán Caisil (qv) as prisoner, as a result of a daring raid into Munster by Muirchertach – before the latter met his death.
Donnchad himself died unexpectedly in 944, the same year as his son Conn, having suffered a brief bout of illness. With the sudden withdrawal of his iron fist, the dynasty lapsed into internecine strife and no fewer than seven short reigns followed in quick succession. His successors included his sons Óengus (d. 945/6) and Domnall (d. 952), his grandson Fergal (fl. 950) son of Óengus, and nephews Donnchad (slain 950) son of Domnall, Áed (d. 957) son of Máel-ruanaid, Carlus (slain 960) son of Conn, and Donnchad Finn (slain 974) son of Áed. Amid all this confusion, Donnchad's nephew by marriage – Congalach Cnogba (d. 956) of Síl nÁed Sláine – was able to make good his claim to the kingship of Tara before that dignity passed to the Northern Uí Néill in the person of Domnall Ua Néill (qv) (d. 980). Eventually, the kingship of the Southern Uí Néill and that of Tara was secured by Donnchad's illustrious grandson, Máel-Sechnaill (qv) son of Domnall, from whom the later Clann Cholmáin kings of Meath descended.