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.
In 822 Cummascach defeated the remnants of this population group, the Ard Ciannachta (in County Louth), at the Battle of Carn Conain and their king, Eudus mac Tigernaig was slain.
[5] In 832 The Vikings plundered Dom Líacc (Duleek) and the sept of the Ciannacht with all their churches.
The Ciannacht were, however, able to eventually win a victory over the Norse and slew their leader Saxolb.
[8] Cummascach died in 839 and is styled rex Ciannactai — King of Ciannachta — in the annals.