Cathal mac Muirgiussa (died 839) was a King of Connacht from the Uí Briúin branch of the Connachta.
His reign coincided with the renewal of Norse raids and in 836 the Vikings made a cruel devastation of all the lands of the Connachta from the Shannon and the west coast.
[3] In 837 the powerful King of Munster, Feidlimid mac Crimthainn (died 847), ravaged Uí Maine and invaded Connacht.
However Cathal threw back this invasion in Mag nAi (a plain in central County Roscommon).
[4] The virtual hereditary kingship that his father had established was broken upon his death but became restored when his nephew Conchobar mac Taidg Mór (died 882) became king c. 855.