The National Library of Ireland announced earlier this year that it would be putting digitized versions of all of its microfilms of Roman Catholic parish registers sometime this summer. Well, that sometime is now, and the registers are online.
This is a very useful tool, but it isn't the first place that most people should look. The information is not indexed, so what one needs to do is find the parish first, then see if the register exists for the time frame in question, then review the individual pages for the information. The pages can then be downloaded.
If one wants indexed records, there is always the outrageously expensive rootsireland.ie ($255/yr! - or just save up all of the searches that you want and buy a single month at $28). I have had less success with searching with FindMyPast.com
Sadly, it is still difficult to do research without knowing the town name where one's ancestor was born, married and/or died.
When I was in Dublin just over a year and a week ago, I reviewed these registers and found one piece of information that I didn't previously know.
Of course, the information that I looked for in Ireland is now online for free - I mentioned this type of thing happening in my previous post about Genealogical Murphy's Laws.