Under JAR, your licence is valid Europewide. It's more an administrative thing than anything. Costs you about 400Euros I seem to think, and takes about 10 weeks (paperwork between CAA and IAA etc). After that, you surrender your licence to the IAA either by post or in person and they give you an Irish one.
It's purely a paperwork exercise - any ratings (SE, Inst, etc) will be transferred to your new licence, and you can continue to fly aircraft on the G register. Be warned though, light aircraft flying is generally not allowed at Ryanair, as you might breach the 900 hours a year mark. I know some guys who fly, but do so in a slightly under the table fashion.
Horgy