Update!!!: See irishgenealogy.ie for an online civil records index. [updated 3rd July 2014]
New on the internet:
An index of Ireland Civil Registrations:
Marriages: 1845-1958 (note non-Catholic records from 1846-1864 only)
Note they are for all Ireland before 1922 after which the indexes are for the 26 counties only.
I found that there is a history of Civil Registration in Ireland, including a map showing the Poor Law Unions (Civil Registration Districts) at GRO Ireland. This might be useful as well as interesting.