There is also a SPA at the Merrion. The area is in the Garden Wing, so you have to go from the Main House through the lobby, down the glass corridor, past the restaurant and then into the basement.
There is a photography ban in the SPA, which is OK for now.

When no one was there but me, I was able to take a few photos. The area is pretty, even if there are no really nice relaxing areas.