There are a number of pay sites for Ireland, some (about 3 offhand) excellent, most are a waste of money. None have anywhere near the quantity or quality of Scotland's People.
Ulster Historical Foundation is excellent but expensive. I joined when they posted the searchable database of all recorded Births and Marriages for five counties of Northern Ireland with the promise that the Deaths were "Coming Soon". That was well over a year ago, my subscription lapsed and no sign of the deaths.
However just to view a transcript at this site costs £6 for members and £8 for non-members. Subscription is about £30 per year. Once you've paid copy the details quickly because you only have access for 24 hours (no "Previous Search").
There are five databases open to the paying public containing well over a million records, members have access to others, some, passenger lists of 16 people to Wills etc. containing upwards of 170,000 records.
Otherdays is another excellent pay site, it has the 1803 North Antrim Agricultural Census and Griffiths Primary Valuation online. Once you've paid your membership you have full, unlimitted access to all the MANY databases for a year.
Irish Times is an online newspaper with some useful genealogical searches available. They also offer paid searches and reports but I can't comment on the quality of these.
These are the ONLY three that I would consider subscribing to.
As with all Irish research it really is important to have a good idea of where your folk came from and their Religion.
There are also a great many area specific FREE sites, too many to list here. Some, Ben Palmer's for Moville and my own for Rathlin Island have near 100% coverage for every available resource for that particular area.
My advice would be identify and exhaust the free options before forking out on an unknown. For example if your lot came from Sligo membership to UHF would be a complete waste of money.
General sites of note include:
http://www.cmcrp.net/
http://www.censusfinder.com/ireland.htm (many are just links to pay site)
There was another free site, now paying, that had searchable index of nearly all Irish Births, Marriages and Deaths, it's now charging. There are no actual records available just instructions of where to find the records (sometimes three or four locations).
If you post the name of the site you're looking at we may be able to advise you further.
Searching for Keogh, Kelly, Fitzgerald, Riddell, Stewart, Wilson, McQuilkin, Lynch, Boyle, Cairney, Ross, King, McIlravey, McCurdy, Drennan and Woods (to name but a few).
Also looking for any information on Rathlin Island, County Antrim, Ireland.