Stonerosefan,
Funny you should ask that question! I am a broker, and am working on a very similar case right now. I have been in contact with all the banks in the last couple of hours on this matter. In short, you can get your fiancee on the title deeds, and both of you on the mortgage with the following institutions: Ulster Bank, IIB (only if you are engaged) Bank of Ireland, and maybe NIB. So far, none of the others have said yes.
I dont know about the tax situation, but from a legal point of view, I would have some sort of agreement in place prior to proceeding, in the (hopefully unlikely) event of a break up.
Does this help?
M