This stone is not on the map so may be modern, but there are no less than four early settlements on this tiny island with numerous hut circles and a promontory fort. RSPB reserve with access by launch from Martins Haven.
I cannot quite remember the exact location as I photographed it while running from a ferocious thunderstorm. Some of the early settlements may be monastic. Local name for this stone is “John’s” stone. Perhaps it is just a cattle scratchpost.