Internal 50mm insulated plasterboard on cavity block

Considering putting internal 50mm plasterboard on the walls of my cavity block built house(cattle shed) the wall consists of a cavity block on the outside with 2by 2 lates nailed on gaps filled with amazing rock wool then cobered with a plastic sheet the qustion I have is will the cavity blocks recieve the hammer in mushroom fixings if I put the sheets on top of existing walls, and wiuld it be better to strip plladterboard off the walls and put new sheets against the block, in this case would I need a water proof membrane bettween new sheets and cavity block???