I can only recommend Powerscourt as I've been twice already and I'm going again in June. It's quite expensive (aren't they all?) but really close to Dublin so handy for me. It's cheaper if you go mid-week and share a room (unless you just want a daytime package).
At the moment Powerscourt have an offer of an extra 25 minute treatment included in the midweek break for May and June (see the website for details). Also, if you book a manicure or pedicure you get a free nail care kit.
The food is fantastic and an overnight stay refreshes you so much it feels as if you've been away for a week. You also get full use of the gym, sauna, jacuzzi, steam room etc etc.
I'd love to hear about other places too.