AFAIK the mountains are about as high as Wales? It looks like there's a 600MW storage scheme in the planning stages. That's about a third of the capacity of Wales's largest system (which is the biggest in Europe) and about twice the capacity of the second system.

It's a meaningful contribution I would have thought.

You can do low(er) head pumped hydro storage [0]

[0] https://en.wikipedia.org/wiki/Ludington_Pumped_Storage_Power...