The story
The first element is apparently cú (“hound”); the second probably means "battlefield", making the whole term "hound of the battlefield".
Etymology from Wiktionary, CC BY-SA 4.0.
Cú Roí
Today it means: A king of Munster and sorcerer who was killed by Cúchulainn and avenged by his son, Lugaid mac Con Roí, who was subsequently killed by Conall the Victorious. Full definition →