I have actually never stayed there, though I have seen it and I do have some good friends who were there about three years ago. I remember their saying they liked it a lot. Anyway, here's a link to the site with all the info you need.
http://www.mercedarios.com/index1.htm The place is very pretty, as are the surroundings, and you're a stone's throw away from Pontevedra, Combarro, and quite close to San Xenxo, Portonovo, the Playa de La Lanzada, La Toja, etc.
From what I could tell, the rooms were pretty basic (they're not rustic, but are a bit monastic from the description!), but the picture looked nice enough. Plus it's got this great old library.
Let me know what you think.