The Uí Néill king lists make Congalach Cnogba of the southern Síl nÁedo Sláine, a kin group long excluded from the succession, Donnchad's successor.
In 943 he defeated the Cenél nEógain and the northmen of Lough Foyle, killing Máel Ruanaid mac Flainn, Muirchertach's cousin.
Sources claiming that Ruaidrí assumed the High Kingship after Donnchad, either alone or jointly with Congalach, are: the Prophecy of Berchán, an 11th-century historical poem presented as a prophecy; the southern, anti-Uí Néill Annals of Innisfallen; and the 12th-century Cogad Gáedel re Gallaib (The War of the Irish with the Foreigners) a wildly successful work of propaganda authored on behalf of the descendants of Brian Bóruma.
He defeated Congalach and his ally Gofraid mac Sitriuc, King of Dublin, and set up camp, probably somewhere between Donaghpatrick and Kells, from which he raided Brega and Meath.
According to the Annals of Ulster, his army defeated an attack by the foreigners of Dublin on his camp on 30 November 950, but Ruaidrí himself was killed in the fight along with his son Niall.