Don't think there is any special 'deal' for converting from FAA to Irish CAA ATPL.
Under the pre-JAR system, you contacted the Irish authority & submitted licences/logbooks etc to get a written assessment of what you needed to do.
Much fewer exams than the UK system required: something like law, met & one or two others.
The catch was that the test must be done in a multi-pilot a/c on the Irish register. I think an SD360 was about the smallest operated in the country when I inquired.
End result was that I did mine in the UK. More exams, but any twin would suffice for the test.