There are no sites for Ireland like you get for English records. There are a few sites which cover certain parts of Ireland but they tend to be very expensive.
Without knowing any details it's hard to suggest where to look so below is just some general information which may help you.
Civil registration of births, deaths and Catholic marriages started in 1864 (from 1845 for other marriages). There will be no certificates before these dates.
For earlier marriages, etc. you need to check for church records (if they exist). To do so you need to know the family's religion and where they lived (parish if not actual townland not just the ounty or 'Ireland').
Unusual surnames will sometimes help find a family, occupations might be useful and often overseas certificates give more clues (for example, if a person born in Ireland died somewhere like Australia or Scotland there will often be lots of important details on the certificates).
If you want to post the names and dates that you have someone might be able to help.
