Cathussach mac Ailello (died 749) was a Dál nAraidi king of Ulaid, in medieval Ireland.
He belonged to a branch of this family that settled in Eilne, a strip of territory located between the Bann and Bush rivers in modern County Antrim, Northern Ireland.
The death of Dub dá Inber mac Congalaig as king of the Cruthin (the title used for them at this time in the annals) is recorded in 727.
[2] Cathussach was killed at Ráith Beithech (Rathveagh, modern County Antrim) probably in the interest of the rival Dál Fiatach who retook the kingship of Ulaid.
[4] This would give a possible reign for Cathussach over the Dál nAraidi from 741-749 if this were true and would explain his placing in the king lists of the Cruthin.