http://registers.nli.ie/
The National Library of Ireland has published the Catholic Registers online for free! Sadly, they don't appear to be indexed but at least they are available. If you don't know the parish your ancestors lived in, be prepared to browse....like in the good old days before you could search for a name on the likes of Find My Past and see masses of relevant material. I'm chomping at the bit. I love the excitement of discovery after spending ages just looking, not knowing if you will even find anything.
It looks like they are scans of the microfilms though I have only looked at one parish.
(I am now placing a 'Do Not Disturb' sign on the front door
)