When in the UK, I'm still obliged to follow any restrictions placed on me/the EI reg aircraft by Irish leglisation, I believe
Wherever you fly you are bound by
most onerous of the rules of the state of licence issue, the state of registration of the aircraft, and the state you're flying in.
So if the IAA let you do something but the CAA doesn't and you're in the UK - you can't.
NB this doesn't apply to a French PPL "VFR on top" because that's a restriction the CAA put on UK-
issued PPLs... (Blimey we haven't had that discussion for a good week or two)