Cu Roi (Cu Rui, Cu Raoi) mac Daire is a king of Munster in the Ulster Cycle of Irish mythology. He is usually portrayed as a warrior with superhuman abilities and a master of disguise possessed of magical powers.

Cu Roi further appears in the episode known as "The Trance of Amairgin", variants of which appear in Recension I and II of the Tain bo Cuailnge.  The episode appears as Aislinge n-Aimirgin ("The trance of Amairgin") in Recension I of the Tain. Having followed news of Cu Chulainn's sustained success in single-handedly opposing the Connacht army, Cu Roi once again appears on the scene, this time to fight Cu Chulainn directly. However, on finding Cu Chulainn weak from the injuries which Ferdiad had recently inflicted on him, he refused to carry out his original plan. Instead he faces the giant warrior poet Amairgin, who in a trance is hurling stones at the Connacht army in Tailtiu, with devastating effects. Cu Roi attacks him in kind and their stones meet in the air. They pause when on Cu Roi's request, Amairgen allows the cattle to go past Tailtiu, but seeing as the passage had become difficult, Cu Roi agrees to withdraw from the contest altogether.  The episode in the Book of Leinster (Recension II), called Imthusa Chon Rui meic Daire (header) or Oislige Amargin (text), offers by and large the same story, but adds more explicit detail, notably on the point of Cu Roi's sense of honour in his encounters with Cu Chulainn and Amairgin. First, Cu Roi explains his refusal to fight Cu Chulainn not only by pointing out the inequality between a physically healthy and an injured warrior, but also by saying that a victory would not be his, seeing as it was Fer Diad who had laid low his opponent. Second, the conclusion of Cu Roi's fight with Amairgin is told from a perspective which highlights the role of honour in his motives. Medb insisted "[b]y the truth of your [Cu Roi's] valour" ([a]r fir do gascid fritt) that he should abandon the competition, obstructive as it proved to be to the progress of the expedition. Cu Roi, however, was determined to persist "till the day of doom" (co brunni bratha) unless Amairgin agreed to stop. (When the matter was settled and Cu Roi returned to his country, Amairgin resumed his attacks on the invading army, explaining that his agreement was with Cu Roi only.)

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why was he fighting the army