Máel Dúin mac Fergusa (died 785) was King of South Brega of the Uí Chernaig sept of Lagore of the Síl nÁedo Sláine branch of the southern Ui Neill.
The high king Donnchad Midi (died 797) had campaigned against Leinster in 780 and then made peace.
The next year in 781 the Battle of the Rig was fought between the Síl nÁedo Sláine and the Uí Garrchon branch of the Laigin and their king Cú Chongalt was slain.
Máel Dúin and his cousin Fogartach mac Cummascaig (died 786) were the Uí Chernaig leaders in this battle.
Máel Dúin is titled King of Loch Gabor at his death obit in the annals.