Cathassach mac Lurgain (or Cathassach mac Fíachnai) (died 668) was a Dal nAraide king of the Cruithne in Ulaid (Ulster). He was the son of Fiachnae mac Báetáin (died 626) a king of all Ulaid and possible high king and brother of Eochaid Iarlaithe mac Lurgain (died 666), a King of the Dal nAraide. He ruled from 666 to 668. In 668 he clashed with the Ulaid (Dal Fiatach) at that time ruled by Blathmac mac Máele Cobo (died 670) and was slain at the battle of Fertas (near Belfast).