It measures five metres north to south and rises just 0.8 metres at its highest point, with its opening facing east and its southern end partially buried under rubble. What makes it worth pausing over is what it almost certainly represents: a fulacht fia, one of the most common yet most quietly enigmatic monument types in the Irish archaeological landscape.

Fulachtaí fia (the plural form) are burnt mound sites, found in their thousands across Ireland and dating mostly to the Bronze Age, roughly 1500 to 500 BC. The typical arrangement involves a trough dug into the ground, a nearby water source, and a mound of fire-cracked stone accumulated over repeated use. The working theory, now broadly accepted, is that stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil, the shattered and blackened stones discarded to one side over time, building up the characteristic crescent or horseshoe shape. What exactly this boiling was for remains a matter of some debate: cooking, textile processing, bathing, and hide preparation have all been proposed, and the answer may well vary from site to site. The boggy, waterlogged ground at Derreencollig, at the valley head of the Coomhola, is exactly the kind of setting these sites favour, low-lying and reliably wet, where water would have been close at hand year-round.