It is going to be a great resource, IF you know the Parish your ancestors lived in, since that is the only indexing that has been done. They do have aa very good map that expands the counties to the parish level and often includes the towns and cities covered. I was lucky, I found my Great Grandfathers baptismal record and with a bit more browsing through the records found his four siblings (I was only aware of 2 others). If you have access to the www.roolstireland.ie website (subscription only except for the Griffiths Valuation) most of their records show the Parish. Then you can browse the original record books for confirmation. I have noticed that the actual dates may be off a day or so, depending on what is recorded - the day of birth or day of baptism.
Eileen