If it's a JAA license, it really doesn't matter whether it's issued by the UK or by the Irish. You can fly any JAA-reg aircraft on it. Also, as long as your medical is a JAA medical, it doesn't matter which country issued it.
The only significant difference IMHO is that according to JAA regulations you can fly VFR-on-top (airspace permitting) on a bare PPL, but the UK ANO places an additional restriction on a PPL preventing you from flying VFR-on-top altogether (regardless of airspace) unless you hold an IMC or IR. When converting to an Irish-issued PPL that restriction is lifted, because for licensing purposes you are no longer subject to the UK ANO. But I don't know if the Irish would place a similar (or maybe other) restriction on a JAA PPL through their laws.
What's the reason for converting from UK to Irish in the first place? It will cost money and will most likely not provide any benefit other than administrative convenience.
I'm still happily flying Dutch-registered aircraft on a UK-issued JAA PPL with a Dutch-issued JAA medical. No problem.