Sometimes listed as Maenofferem, but mostly Maenofferen in old newspapers. Fferem is not farm. Fferm is Farm! Offeren as mass is fine , but as mass of stones, not ecclesiastic (The area maybe holey but ,perhaps, less holy)
A mass of stone seems to make more sense.
I hope that they don't use Mynydd Llechi, as Tomen Llechi (slate heap) would be more apt.
Maen means stone . Craig is rock.