On the face of it I can't see why not assuming that she will actually be paying rent and you can account for this if necessary (i.e. you don't just tell Revenue that this is the case even if it's not). After all, in the eyes of the law/Revenue, you are technically unrelated strangers as an unmarried albeit soon to be cohabiting couple. Of course I could be wrong as I am not a tax expert. The easiest way to find out might be to inquire directly with Revenue about this.