When researching my wife's Irish family we found it was best to go to Dublin and visit the Registrars office in person. I think the office has moved since we visited about 6-7 years ago, but I believe the process is the same: you buy credits to see the Index books and then, when you find a possible match, you submit a request for a photocopy of the record which takes them about 15mins or so to do.
I found it much easier than the records office in London where you could only buy a copy certificate and this needed to be posted out.
Regards
Andrew