This site researching Ireland is jam packed full of goodies for the researcher.
The 1901 census is complete for all of Rosscommon, Leitrim, Mayo and Sligo. All of the other counties are still under development but are still online for limited searching.
Other databases include Surnames, Townlands, Griffiths valuation. This site is also updated on a regular basis. You will find a link to The Leitrim -Rosscommon Genealogy Web site here.