The IAA don't recognise FAA class 3 medicals, believing them to be sub ICAO. They are perfectly happy though with an FAA class 1 or class 2 medical.
Is this for conversion, or to fly EI aircraft on an FAA certificate in Ireland ?
Irish law automatically validates all ICAO licences for use on EI reg aircraft for private day VFR flight (without geographic restriction, though there is a confusing AIC which tries to limit it to Ireland).
Obviously any such licence needs to be accompanied by an ICAO level 2 medical or above (the standard required for private flight licences under ICAO).
The IAA have taken the view that an FAA class 3 medical is not an ICAO level 2 medical or above, and therefore cannot be used as the basis for flying an EI reg aircraft on an FAA PPL.
dp