IrishGenealogy.ie has a freely searchable online version of the GRO's own digital index up to 100 years ago for births, 75 years ago for marriages and 50 years ago for deaths. There are full images of all these birth records, the marriage records starting in 1882 and deaths in 1891. Wonderful.
Unlike the FamilySearch copy (see below), these birth indexes show the mother's maiden name from about 1900 and marriage records include both parties' names from 1880.
FamilySearch: The Latter-Day Saints (Mormon) website includes a partial transcript of Irish birth records 1864-1881, as well as a complete transcript of their microflims of GRO indexes up to 1922 (all-Ireland) and 1958 (Republic of Ireland). Ancestry and FindMyPast.ie each a copy of both.
Eleven of the heritage centres whose records are at Rootsireland (€) have transcribed at least some local Superintendent Registrar's registers, usually up to 1921. Some areas (e.g. Donegal) are complete.
All historic GRO records within the six counties now part of Northern Ireland (births to 1916, marriages to 1941, deaths to 1966) are searchable online at https://geni.nidirect.gov.uk (€).
Assorted: Various individuals and organisations have transcribed records covering their area of interest.
IrishGenealogy.ie has images and transcripts of Roman Catholic and Church of Ireland records for Carlow, Cork, Dublin city and Kerry.
Ancestry.com (€) transcribed 46 Roman Catholic parishes from National Library microfilm records, mainly in the diocese of Meath, (2013).
Ancestry.com (€) also has fresh record images and transcripts of 71 other Roman Catholic parishes (2014).
Registers.nli.ie is the National Library of Ireland's online Catholic parish register microfilm site. All the records are untranscribed and almost all end in 1880 or before.
Approximately 85% of Roman Catholic records before 1900, 60% (?) of Church of Ireland records and an increasing proportion of Presbyterian and other denomination records are transcribed at rootsireland.ie (€).
Ancestry.com (€) and Findmypast.ie (€) have collaborated to make a full transcript of the Registers.nli.ie Catholic parish register microfilms.
The Representative Church Body Library has records of fourteen Church of Ireland parishes available for download at the Anglican Record Project
Assorted: Various individuals and organisations have transcribed church registers covering their area of interest.
FindMyPast has a complete set of the records of the Dublin Quaker Library.
Findmypast.ie (€) has a complete set of Griffith's Valuation (1847-64), (€).
A complete set of Griffith's Valuation (1847-64) is available free at the former Irish Library Council site, www.askaboutireland.ie.
Post-1858 District Will books to 1900 are also online, along with most surviving pre-1858 will books. FindMyPast has a free, better-organised version.
The Public Record Office of Northern Ireland website has the Calendars of Wills and Administrations for areas now in Northern Ireland from 1858 to 1965. Early records are linked to images of the surviving pre-1922 transcripts of full wills for Armagh, Belfast or Londonderry district registries.
Discover Ever After is based in Northern Ireland and provides a cemetery records management system to churches and local authorities including, in most cases, on-line headstone transcripts and cemetery maps. So far they have worked mainly in Ulster, also covering some cemeteries in north Connacht and north Leinster.