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.
It is gorgeous cava, really lovely. They have it on the wine list in Rhodes D7 and Bar Pintxo in Temple Bar. They also sell it to take away in Bar Pintxo ... but it is quite pricey and I want a few bottles for a party.
"Anna" by Cordoniu is my other favourite cava but I have to say Monistrol just pips it
Mc Hugh's off-licences in Kilbarrack Road or Malahide Road in Artane have Marques de Monistrol on special offer at the moment. They have both the Brut Reserva and the Vintage for €10.95 a bottle.
Is it one of these that you liked? http://www.mchughs.ie/