I would recommend Blue Daisy - it is on Oliver Plunkett Street and the entrance is next door to the Old Oak pub. No affiliation - just a satisfied customer. They do a wide range of treatments and you should find something to suit your budget.
Depending on what you are actaully lokking for, there is a really nice spa in Model Farm Rd, Aquaspa that have a huge variety of treatments and spa days. Also Sam Mc Cauley in Douglas.....lovely Guinot products....really nice facials