It is, in fact, a fulacht fia, a type of prehistoric cooking site found widely across Ireland and dating most commonly to the Bronze Age. The usual interpretation is that these were places where water was heated by dropping fire-cracked stones into a trough, then used to cook meat, though theories about other uses, including bathing or brewing, have gained some traction among researchers. The characteristic shape here gives the game away: a horseshoe-shaped mound, roughly twelve metres north to south and just under ten metres east to west, rising about a metre at its eastern edge, with a hollow at its centre opening to the west.

The site sits on the edge of a limestone terrace, with hazel scrub advancing from the north and a waterlogged, boggy area directly to the south. That proximity to wet ground is entirely typical. Fulachtaí fia are almost always found near a reliable water source, since the whole process depended on a ready supply. The exposed stone visible at the eastern side is grey-black and friable, the burnt and shattered material that accumulates over repeated use and gives these mounds their bulk. Over time, as cracked stones were discarded after each heating cycle, the spoil built up into the distinctive raised ring that survives today at Keelhilla.