Sand person are you sure that rainwater is not used for fresh water? All it needs is clean collection and filtering. Grey water, I am given to understand, is water that has already been used for cleaning and washing, and is then used for flushing. Or can be locally filtered and re-used for more washing, etc.

Desalinization requires a considerable amount of energy and maintenance. Given that Ireland has considerable annual rainfall, as I know personally, it would make sense to determine locations of storage reservoirs and so build them. When you don't need to generate the megawatts, it is the same as if you needed to build the generation plant and then didn't build them. Those are called NEGA watts.