He peacefully succeeded to the throne of Osraige upon the death of his older brother Cerball mac Dúnlainge in 888.
[2] He was also brother to the influential princess Land ingen Dúngaile.
William Carrigan states that Riagan must have been aged upon ascending the throne, as his brother's famous reign lasted over forty years.
The Annals of the Four Masters record him winning a victory over the Vikings of Waterford:[3] FM888.6: A battle was gained by Riagan, son of Dunghal, over the foreigners of Port Lairge, Loch Carman, and Teach Moling, in which two hundred heads were left behind.Whether king Riagan died or abdicated on account of age is not known; he was succeeded by his nephew Diarmait mac Cerbaill.
This biography of a member of an Irish royal house is a stub.