Do I need to have CA with same Bank as my my Mortgage?

I recently closed my AIB CA due to fees and now do my banking through BoI as I had a CA with them which was set up when I got a mortgage from them.

I'm none t0o happy now that BoI are also introducing fees. Theoretically, can I close this account or do I need an account with Boi as my mortgage is with them?

No, unless it's a condition of your mortgage (and I don't believe BOI had products with that requirement)