Previous men reckoned High King had belonged to the great Uí Néill kindred, that large group of families who traced their descent from Niall of the Nine Hostages, which dominated much of central and northern Ireland from the 7th century onwards.

His year of birth is unknown, but a date in the 980s is likely, as it would accord with the political policy of his father at the time and mean that Donnchad was old enough to be militarily active in the 1010s, when he first appears in historical records.

Toirdelbach first joined with Áed in the early 1050s, raiding into Tuadmumu in 1052 and inflicting a heavy defeat on Donnchad's son Murchad in Corco Mruad, the north-west of modern County Clare in 1055.

This story is repeated in many 19th century and earlier works of popular history and is given as an explanation of how the English Pope Adrian IV came to issue the papal bull Laudabiliter granting rule of Ireland to King Henry II of England.

Keating writes that "this story cannot be true, for before setting out on that expedition [Donnchad] was a very old decrepit man of over eighty years of age, and it is not likely that an emperor's daughter would covet intercourse with such a veteran".

His grandsons Conchobhar and Cennétig, sons of Lorcan, became kings of Tulach Óc in east Tyrone and operated against their cousins in the 1070s and 1080s.