Donnchad was the son of High King
Flann Sinna by his wife
Gormlaith ingen Flann mac Conaing. The date of his birth is not known, but he was apparently an adult in 904 when he is found as the leader, or figurehead, of a challenge to his father at
Kells. The
Annals of Ulster state that many of Donnchad's associates were beheaded by Flann, and that the High King profaned the sanctuary at Kells to seize Donnchad. Donnchad again rebelled against his father in 915, with the support of his brother Conchobar, but this rebellion was suppressed by his sister Gormlaith's husband, and his father's heir,
Niall Glúndub. When Flann died in 916, Niall Glúndub succeeded him as King of Tara, while Conchobar became
King of Mide, the kingship of Clann Cholmáin. Niall and Conchobar were both killed on 14 September 919, in battle against the Foreigners—
Vikings and
Norse-Gaels—at
Dublin. Other Irish kings and princes among the dead were another of Donnchad's sister's husbands, Máel Mithig mac Flannacáin, Niall Glúndub's brother's son and heir-designate Flaithbertach mac Domnaill, "and many other nobles". Donnchad succeeded to the kingship of Mide, and to the high kingship of Ireland. He immediately had his brother Áed blinded. Some years later, Donnchad disposed of his nephew, Máel Ruanaid, Conchobar's son. ==Reign==