Cummascach mac Congalaig (died 839) was a King of Brega from the Uí Chonaing sept of Cnogba (Knowth) of the Síl nÁedo Sláine branch of the southern Uí Néill. He was the son of Congalach mac Conaing (died 778) and brother of Flann mac Congalaig (died 812) and Cernach mac Congalaig