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.

Later, Blathnat (Blanaid) betrays Cu Roi to Cu Chulainn, who besieges his fort and killed him. In one version of the story, Cu Roi's soul was hidden in an apple in the belly of a salmon which lived in a stream in the Slieve Mish Mountains, and only surfaced once every seven years; Blathnat discovered the secret and told Cu Chulainn, who killed the fish, enabling him to kill Cu Roi. However Ferchertne, Cu Roi's poet, enraged at the betrayal of his lord, grabbed Blathnat and leaped off a cliff, killing her and himself. Cu Roi's uncle (or brother or nephew), Conganchnes ("Horn-skinned"), tried to avenge him, but was killed by Celtchar. His son, Lugaid mac Con Roi, later succeeds in avenging him by killing Cu Chulainn, a story told in Aided Con Culainn. Lugaid is himself killed by Conall Cernach. In another version Cu Roi takes Blathnat to the fort and keeps her captive there. Blathnat communicates with Cu Chulainn and a plan is hatched. Taking an opportunity when most of Cu Roi's men are absent from the fort, Blathnat gives the signal to Cu Chulainn by pouring milk into the Fionnghlaise (white stream - now the Derrymore River). Cu Chulainn, on seeing the stream become white, storms the fort, kills Cu Roi, and carries off Blathnat. As Cu Roi's men return up the valley, Blathnat places a spell which makes the valley walls dance in front of the men's eyes. Walkers who ascend Caherconree via the Derrymore River valley can still see this effect which is caused by an optical illusion.

Did con roi survive the fighting?
Blathnat discovered the secret and told Cu Chulainn, who killed the fish, enabling him to kill Cu Roi.