What a wonderful piece of news, congratulations. Sandra has provided you with a good start there, It is always best to begin by sorting out the legality of it all.This article goes into everything you need to know in a bit more detail.
There is also a link at the bottom of that page to some of the more popular venues, including castles. You can jump to that here
. I realise with your plan to elope, they may be a little larger than you require, but certainly worth a look.
The law has actually changed in Both the UK and Ireland. So you can now get married outside, no matter where on the Island you want to do it. Your castle ruins are therefore definitely an option.
You mentioned you would like some other suggestions, have you thought about a hole-stone. These are standing stones with a hole carved through them. Traditionally there is a handfasting ceremony (ancient wedding ceremony) where you pass a ribbon through the hole and bond the hands of the couple together. Essentially binding them in spirit, earth and heavens. It is worth a thought, there are many stone circles that are popular for such ceremonies too. If you want more info on this let me know as I have locations for quite a few of them, from planning my own wedding.
I have to ask, why Rahinnane Castle? It is a beautiful old ruin, and I totally get the aesthetic, but with it being such an obscure choice I can't help but wonder if you have some kind of connection to the castle.