Cernach mac Fergusa (died 805) was a King of in South Brega of the UàChernaig sept of Lagore of the SÃÂl nÃÂedo Sláine branch of the Southern UàNéill. He was the son of Fergus mac Fogartaig (died 751) and brother of Máel Dúin mac Fergusa (died 785) and Ailill mac Fergusa (died 800), previous kings.
Cernach succeeded his brother Ailill as King of Lagore but the rule of South Brega went to a subsect of the UÃÂ Chernaig, the SÃÂl Conaill Graint which were based at Calatrium. He ruled as King of Lagore from 800 to 805. At his death notice in the annals, he is called rex Locha Gabor -King of Lagore. The title King of Loch Gabor was used when members of this sept were not Kings of South Brega (instead held by the sub-sept SÃÂl Conaill Graint).