It is the kind of thing that could easily go unnoticed, not through any lack of presence but through sheer inaccessibility. Green Island holds one of the quieter standing stones in County Mayo, and its obscurity is almost entirely a matter of geography.

The stone itself is a roughly rectangular granite slab, about 1.1 metres high and a metre long, set upright in the ground with its long axis running east to west, leaning gently to the south. It tapers noticeably from west to east, thickest at around 0.3 metres on the western end and narrowing sharply toward the east. The north face is relatively smooth and flat; the south face is slightly convex and rougher in texture. Standing stones, which are prehistoric upright stones erected singly or in groups, are found across Ireland in great numbers, though their original purposes remain genuinely uncertain. This example sits toward the north-west end of the island, and its north face carries a scattering of small pits and indentations that might, in other circumstances, suggest carved markings. They are not. They are natural features of the granite, a reminder that stone has its own surface history quite apart from anything human hands might have done to it.