Niall mac Conaill (died 778) was King of South Brega of the Uí Chernaig sept of the Síl nÁedo Sláine branch of the southern Ui Neill.
Upon the death of Coirpre mac Fogartaig of the Uí Chernaig sept in 771 the rule of Brega was recovered by the northern sept of Uí Chonaing based at Cnogba(Knowth) in the person of Congalach mac Conaing (died 778).
Niall represented a branch of the Uí Chernaig sept called the Síl Conaill Graint which were based at Calatrium (Galtrim), whereas the main line of Uí Chernaig represented by the sons of the high king Fogartach mac Néill (died 724) were based at Lagore.
[2] The sons of Fogartach had dominated southern Brega since 737 and a claimant arose in Cummascach mac Fogartaig to challenge Niall's rule.
In 777 the Battle of Calatrium was fought between Niall and Cummascach in which the vassal king Échtgus mac Baeth of the Deisi Brega was slain.