It measures roughly two metres east to west and a metre and a half north to south, rising only about sixty centimetres from the ground. What lifts it out of the ordinary is what is carved into its slightly southward-sloping upper surface: prehistoric rock art, the kind of deliberate marking made by people in Ireland somewhere between the Neolithic and the early Bronze Age, whose precise meanings remain genuinely unresolved.

The carvings are faint, worn down by millennia of exposure to the Kerry weather. The clearest element is a picked line, traceable for around half a metre, running northeast to southwest across the surface. Rock art of this kind was made by pecking or grinding into stone with a harder tool, leaving grooves and depressions that were presumably meaningful to whoever made them, though whether they functioned as territorial markers, ritual surfaces, or something else entirely is still debated by archaeologists. At Derrynablaha, whatever further markings once accompanied that traceable line have become extremely weathered, their forms now only barely legible on the stone.