Have never found Monistrol - have to settle for Freixenet which isn't bad value in Tesco (comparatively-speaking for this country)
There is an off-licence in Rathmines opposite the Tesco store, where I have gone to get Freix snipes, which is the only place I've ever managed to find these. I've always found the proprietor v.knowledgeable and I'm sure she'll tell you once and for all if you can get Monistrol here or not.