It is not natural. This is a fulacht fia, a type of prehistoric cooking site found across Ireland in the thousands, and its modest profile, roughly ten metres along its longer axis and less than a metre high, conceals a fairly specific piece of ancient domestic engineering.

A fulacht fia typically worked by heating stones in a fire and dropping them into a water-filled trough until the water boiled, allowing meat to be cooked by immersion. Over time, the cracked and spent stones were raked out and piled around the trough, forming the characteristic horseshoe or crescent mound that survives long after everything else has gone. At Keelhilla, the mound encloses a damp central hollow roughly half a metre deep, still wet and soggy at its open northwestern end, suggesting the water source that would have made this spot functional in the first place. The mound itself is built from a dense concentration of limestone fragments set in dark grey and black soil, the darkened earth a likely trace of prolonged burning. The site sits within the Keelhilla and Slieve Carran Nature Reserve, part of the Burren National Park, overlooked to the west by the steep cliffs of Eagle's Rock on the eastern face of the Slieve Carran ridge. The broader depression, about thirty metres across, is ringed at its southern, western, and northern edges by raised rock outcrop, which would have given some shelter from prevailing winds and made it a naturally attractive place to stop and cook.

The Burren's karstic limestone landscape, where rainwater drains quickly through fissures in the rock, makes the persistent dampness at this site all the more notable. The wet hollow and soggy ground to the northwest point to a localised water source in an area where surface water is otherwise scarce, and that reliable wetness was almost certainly why someone chose this precise spot, perhaps four thousand years ago, to build their fire and pile their stones.