Do the Celtic and Norse peoples remain enemies throughout The Destiny Cycle?

No. The Cycle begins amid Norse occupation and Gaelic resistance, but that relationship changes. By Warriors of Destiny, Breanna's blood-bound peace with Odin and Loki extends to Norvegrs, Gaels, and Celts. Odin accepts Breanna's invitation for his seaworthy Norvegrs to join the North's resistance against Rome.