Well, the Roman Catholic registers haven't been online for long but I've already made my first discovery! Granted, I knew the parish, & the townland, & it was only a few pages into the microfilm, but I've managed to translate most of the entries. Some of the other siblings will be just as easy to find, but some of the siblings were born before this register started in 1849. Finding other Irish relatives won't be anywhere as easy because at best I have a county, & one I just have 'Ireland'.
Here is my first discovery:
5 Dec 1850 - Bap Hannam f Michaelis Curran + Anna Mulheron de Derryreel
Translated 5 Dec 1850 Baptism of Hannah, daughter of Michael Curran & Anna Mulheron of Derryreel.