The Merrion Hotel would be the most luxurious option, and it's very convenient to St Stephens Green and Grafton Street.
La Stampa Hotel has a wonderful spa and is right on Dawson Street so you can't get much more central than that.
Enjoy!
Ah I'm jealous!
I was given a voucher for the spa there a while ago and I have to say it was one of the nicest spas, and best treatments I've ever had.
Have a great time!