Donnchad continued his father's support for the Columban churches, led by Iona. In his many wars he used the churches, particularly the Columban monastery of Durrow, as a source of support. He also ruthlessly attacked and plundered those churches which supported his rivals among the Uí Néill and also those of Leinster and Munster. Donnchad was remembered, not always fondly, as a warrior king. He firmly established Clann Cholmáin's dominance among the Uí Néill kindreds of the midlands. His descendants shared in the High Kingship until the time of Máel Sechnaill mac Domnaill, the last traditional High King of Ireland.
Donnchad's distant kinsman Fallomon mac Con Congalt of Clann Cholmáin Bicc appears to have held the title king of Mide at his death in 766, so that Donnchad can at most have been king of Uisnech, chief of Clann Cholmáin, at his father's death. The Irish annals record strife among Donnchad's kin after his death. His brother Diarmait Dub was killed in 764, leading the forces of the monastery of Durrow in battle against those of Clonmacnoise, led by Bressal mac Murchado, probably his brother's son. Bressal was himself killed later the same year. That year Donnchad defeated the Fir Tulach Midi, a minor people who lived by Lough Ennell and the following year, with the support of Fallomon mac Con Congalt, he defeated and killed his own brother Murchad at Carn Fiachach, near present-day Rathconrath, County Westmeath. Fallomon was killed in 766, after which Donnchad became King of Mide.
The late Annals of the Four Masters places Niall Frossach's abdication in the same year as Donnchad's campaign in Leinster, dated to 770 by the Annals of Ulster, and places the beginning of Donnchad's reign from 771 AD. Later sources present the succession of High Kings as regular, with one king following another immediately. This is not believed to be an accurate representation. Where Niall Frossach is concerned, some years may have elapsed between the death of Domnall Midi and Niall's inauguration. Another possible cause for, or sign of, Niall's fall from power appears in 771 and 772 when Donnchad campaigned in the lands of the northern Uí Néill.
In 775 Donnchad took control of the monastery at Clonard in the Leinster borderlands. He also campaigned in Munster. The Annals of Ulster record that Donnchad "did great devastation in the territory of the Munstermen, and many of the Munstermen fell". He repeated this in 776 with the aid of the community of Durrow.
Donnchad is recorded as twice having disturbed the óenach of Tailtiu, first in 774, when no explanation is given, and again in 777, this time the annals state that the Ciannachta, by which the Síl nÁedo Sláine, this time the north Brega branch of the kindred, are meant, were the targets. This is explicitly linked to the war between Donnchad and Congalach mac Conaing, the King of Knowth, which had begun earlier in 777 when Donnchad led an army from Leinster into Brega. A pitched battle somewhere in Brega in 778 ended with Congalach and many of his allies dead.
A raid on Donnchad's territories by the Leinstermen in 780 was repulsed. Later in the year a meeting between the Uí Néill and the Leinstermen was held at the King of Tara is believed to have settled whatever dispute had provoked the raid. In 784 a similar meeting appears to have been planned between Donnchad and Fiachnae mac Áedo Róin, the King of Ulster, at Inis na Ríg, one of the islands which given modern Skerries its name. The intended conference was turned into a non-event by Donnchad's refusal to appear the lesser party by boarding Fiachnae's ship and Fiachnae's refusal to come ashore for the same reason. This was commemorated in verse in the margins of the Annals of Ulster.
In 786 the annals record that Febordaith, head of the monastery at Dulane, was killed. A later gloss adds that the killing was avenged. This appears to be related to the following entry in the Annals of Ulster, which reports that Donnchad defeated the Síl nÁedo Sláine at Lia Finn, near to modern Nobber, killing Fogartach mac Cummuscaig, the king of Lagore.
In 791 Donnchad is said to have "dishonour[ed] the staff of Jesus and relics of Patrick" during an óenach, probably the óenach of Tailtiu. The óenach Tailten may have seen further trouble in 791, for Donnchad attacked Áed Oirdnide and drove him from Tailtiu and out of the valley of the river Boyne. Cathal mac Echdach, king of the Uí Chremthainn, and other notables were killed in the rout. The last of the many records of Donnchad at war comes in 794, when he aided Leinster against Munster. Donnchad died early in 797, aged 64. He was succeeded as High King by Niall Caille's son Áed Oirdnide and by his son Domnall as head of Clann Cholmáin and King of Mide.
Donnchad's reputation was mixed. The Félire Óengusso, written at Tallaght in the borderlands of Leinster, apparently includes him among the oppressive secular rulers whom the authors dismissed as at best unimportant and at worst wicked. It does, however, confirm the apparent record of the annals, that Donnchad was a warlike ruler quite unlike his father, referring to him as "Donnchad the wrathful, ruddy, chosen". While Donnchad was a friend to the Columban churches, others religious communities, and especially those which lay on the borders of Munster, suffered at his hands. Although earlier histories saw the arrival of the Vikings, first attested in the seas around Ireland shortly before Donnchad's death, as responsible for changes in warfare which made churches a frequent target, Donnchad and his contemporaries sacked churches with some regularity.
Conn had predeceased his father, killed in 795. It was Domnall who succeeded Donnchad as King of Mide on his death, but he ruled for only a short time. The Annals of Ulster report that "Domnall son of Donnchad was treacherously killed by his kinsmen" in 799.
Donnchad's son Conchobar mac Donnchada was later king of Mide and High King. Máel Ruanaid was king of Mide and father of later High King Máel Sechnaill mac Máele Ruanaid. Ailill was killed in 803 fighting against his brother Conchobar at Ruba Conaill. Óengus died in 830, he is called "king of Telach Midi"; so too did Fallomon, killed fighting the Munstermen. It may be that Ruaidrí son of Donnchad, the secundas abbas of Clonard and tanaise of Clonmacnoise—these terms probably mean he was vice-abbot of both communities— whose death in 838 is reported by the Chronicon Scotorum, was a son of this Donnchad.