Yes I think it's pretty much as 8AS says. Pilots are probably recruited on personal recommendations from other pilots. I guess if you have experience of something similar somewhere you have a better chance.
Actually you don't really have to be Irish. I have seen English and French pilots work for them. In fact their first pilot was British. He wrote a fascinating book about the subject. Ironically I think it may be harder to get into the Islander operation than the ATR's.