To answer the question, your PPL only becomes an ICAO PPL, transferable between countries, once it's a PPL. Until then, it's country-specific bits of paper.
So if you did the ground exams with (of) the UK CAA, then you need to do the skills test (and the R/T practical) under the auspicies of the UK CAA too, and send in the paperwork so that the CAA can issue you the (JAR-FCL compliant) PPL. Only then can you transfer that PPL to the IAA.
Alternatively you simply forget about the UK CAA ground exams you did, and do the whole set of IAA ground exams - that should not be too difficult now. Your experience hours are just that - experience hours, so they will be recognized by the IAA too. Then all that remains is the IAA skills test and you've got an IAA PPL straight away.
But since you're going on with CPL etc: Did you know that *any* ICAO PPL (UK CAA, IAA, FAA, whatever) is good enough as an entry for the CPL? It doesn't have to be from a specific country. And a JAR-FCL compliant PPL is recognized and validated implicitly by the IAA (and vice versa) so for your training up to CPL it really doesn't matter where the PPL was issued, as long as this was in a JAA state.