An erect monolith, 1.7m high by 0.8 by 0.5m: one of several stones in SW Wales bearing the same name, which seems to derive from Gerald of Wales, who said that the stones were erected by Earl Harold to commemorate his victories over the Welsh in 1063: the relation between this stone and the extensive settlement remains on the island is unclear.
Source: J Wiles 23.05.02 Royal Commision AHM Wales
Whether ancient or not, it certainly is enigmatic on a foggy day!
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.