Yes, I am referring to British pounds, not euros. I'm just pointing out that it is indeed possible for money to become "outdated" and no longer accepted - and that this can happen quickly. I was caught by surprise (at least I learned about this online, so I knew how to handle my old money without losing value).
Your euros are fine, even if the designs have changed.
And yes, since Northern Ireland is part of the UK, it uses British pounds (GBP), not euros (EUR). I see the Heart of Ireland tour does not go to Northern Ireland, but if you're visiting there before or after, you will be using GBP for any cash needs there.