I recently went to Cork for a spa day in the Rochestown Park Hotel with a friend.
The spa day started at 10am, finished at 5pm, included 4 or 5 treatments with full use of the leisure facilities and relaxation lounge. I'm looking for something similar but in the Kildare area.
I havent been to any of them, but I have been to www.powerscourtsprings.ie - not in Kildare but they do the kind of day spa package you are looking for.
Marriott Johnstown House Hotel & Spa in Enfield, Co Meath is really nice. (Its pretty much on the Kildare/Meath border.) They do spa deals and you have the use of the leisure facilities as far as I know!
If you were prepared to go a tiny bit further, I'd highly recommend the Spa at the Heritage Hotel at Killenard (not to be confused with the one in Portlaoise), it's about 10 minutes or so away from the Monasterevin turnoff. Their Spa Experience area which is included in their "signature" treatments is really, really nice. You can also do this separately for €60 for 3 hours.
I have been to KIllashee House Hotel.Brooklodge in Wicklow,Aghadoe Heights,Killarney,Russel Court,Cavan all with spas. Killashee was superb.Fabulous treatment rooms.Staff gave me a tour of all the treatment rooms as I am a massage therapist myself and have toured to see as many spas as possible.The only draw back with Killashee is that the spa is in a separate building to the back of the hotel.However hotel has got real character.