If you are searching for Irish ancestors, you may want to check out the National Archives of Ireland. The site offers a free search of their archives, which include records from various government agencies and departments, as well as a handful of miscellaneous databases and lists.
Depending on the database, you may be able to find information as far back as the late 1700s. With a couple exceptions, the records on the site are not available for viewing. A search simply gives you a brief description of the record. However, it does appear that you can order the records through the mail for a fee.
The archives have a page set up to aid you in locating records with genealogical relevance. The only two of these records available online currently are the 1911 Census and the Ireland-Australia Transportation Records.