Hi Marie,
I use Roots Ireland a lot, especially helping out answering questions in G2G here. I've found Roots Ireland very reliable, slightly expensive for getting old certificates. In Ireland birth records date from 1864, when the Irish civil registration system was introduced. Until this time, a child's arrival was recorded only by baptism (or christening, according to religious denomination).
There is one major glitch with older records, which affects not just Roots Ireland but most if not all genealogy search sites. The glitch is that pre 1785 a lot of the records were recorded in Latin and if you search for the Anglicized/English name it won't return a result. To get the result you have to enter the Latin version of the name.
Another problem for Irish records is some villages actually straddle several Parishes and dependent on where the person who was recording the event was working the records could/can be recorded in different Parishes (it could just be a case of the person crossing the street for this to happen).
If you need any more help feel free to contact me.
All the best
Billy