Tried and tested:
Ferrycarig Hotel, Wexford, Love it, go for the Junior suite though...
Cliff House hotel in Ardmore, absolutely breathtaking location. We had the 8 course tasting menu.
Farnham Estate in Cavan (absolurtely amazing place) Huge grounds, lakes on the property for walks, you can borrow bicycles..the spa is ...great!!!! They have loads of DIY treatments that are free and obviously the treatments that you can book. Was there a month and a half ago and have already booked my next stay. Loved it.
Or for something completely different, there is a spectacular self catering alternative: Trag retreat in West Cork (There is an outdoor hot tub) This is a very special place indeed....