Fearghal mac Catharnach (died 821/823) was Lord of Loch Riach.
He appears to have been a son of Cathrannach mac Cathal, described as of Maenmag in the Annals of Ulster upon his death in 801.
Loch Riach is a lake at the foot of the Sliabh Eachtaí, located in a territory then called Máenmaige; its ruling dynasty were the Ui Fiachrach Finn.
In 802, The demolition of Loch Riach was undertaken by King Muirgius mac Tommaltaig of Connacht.
A later lord of Loch Riach was Cormac mac Ceithearnach (died 881).