Following on from the revised sticky (the old one is still there but it's unstickied so you'd need to do a search), I thought we'd have an ongoing thread for links. The below list is not exhaustive: it's just meant to be a starting point, and it will include the obvious ones mentioned in the sticky.
1901/1911 censuses (free)
Civil bmd index from inception to 1958 (or 1922 for NI)
RC/CoI/Presbyterian parish records for selected areas & civil registration indexes for 32 counties limited by 100/75/50 rule (free)
RC/CoI parish/transcribed civil records for selected areas (subscription)
Griffiths Valuation (free)
Excellent site with a lot of miscellaneous records
National library for details of what they have onsite, with online catalogues plus registers.nli.ie for their digitised parish register collection.
Limerick City Library - huge pile of helpful stuff online like obituaries, cemetery records, trade directories, electoral register, specific to Limerick obviously.
Commonwealth War Graves Commission
A genealogy forum site with county specific boards.
Current (2015) best collection of Irish records online (subscription)
The original commercial genealogy site: records for Ireland are mostly duplicates but records for US/Australia and UK are great. (subscription)
Claire Santry's comprehensive guide to records published online for the last 5 years. Well worth the cost.