Irish Ancestors is a site that I suspect could be useful to some—especially to those with deep pockets. All the usual suspects are here—census returns, civil registration records, parish registers—and also such valuable census substitutes as Griffith's Valuation and the Tithe Applotment Books. Unfortunately, few of the actual records seem to be online, and there's no guarantee that requests for offline research will produce useful results. Among the free resources are a civil parish map, and a placename database. Try also the Surname and Ancestors searches to see what relevant resources are available, and what a thorough search will cost.