Have you considered the Royal Marine hotel in Dun Laoghaire? I had a treatment at their spa recently and was well impressed. Free use of the thermal suite is included with any treatment. Here's a link to their .
I would also highly recommend the spa at the Heritage hotel, Killenard (near Portarlington. It's only an hour or less from most parts of Dublin (and you can have a little stop off at the Kildare Village shopping outlet). Their signature treatments include the Heritage Spa Experience, which adds up to 3 hours onto your treatment time. Here's the [broken link removed].