Donnchad mac Domnaill (733 – 6 February 797), called Donnchad Midi, was High King of Ireland. His father, Domnall Midi, had been the first Uí Néill High King from the south-central Clann Cholmáin based in modern County Westmeath and western County Meath, Ireland. The reigns of Domnall and his successor, Niall Frossach of the Cenél nEógain, had been relatively peaceful, but Donnchad's rule saw a return to a more expansionist policy directed against Leinster, traditional target of the Uí Néill, and also, for the first time, the great southern kingdom of Munster.
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 churches that 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.