It measures roughly five and a half metres across and just under four metres deep, and to most eyes it would pass without a second glance. But the shattered stones buried within it, cracked by repeated heating and rapid cooling over many centuries, mark it as a fulacht fia, one of the most numerous and least understood monument types in the Irish landscape.

Fulachtaí fia are prehistoric cooking sites, found in their thousands across Ireland, typically Bronze Age in date though some may be earlier or later. The principle was straightforward: stones were heated in a fire, then dropped into a water-filled trough to bring the contents to a boil. The repeated thermal shock eventually split the stones beyond further use, and they were discarded into a mound around the trough. Over time these discarded heaps built up into the low, horseshoe-shaped ridges that survive today, usually in exactly the kind of low-lying, waterlogged ground that made it easy to maintain a full trough. At Barnacahoge, the southern side of the mound encloses a shallow hollow approximately 1.8 metres wide, its position given away by a thick growth of rushes, which thrive in the persistently damp ground where the trough once sat. The charcoal-rich, dark soil visible within the mound is the residue of those long-extinguished fires.

The site sits close to a stream in a valley with steep sides rising to the west, and two field walls, one running north to south between the mound and the water, one running east to west along its southern edge, now cut across the immediate surroundings, folding the ancient site into the pattern of more recent agricultural land division. The rushes marking the hollow are perhaps the most reliable thing to look for; in wet boggy pasture of this kind, that patch of dense, vigorous growth tends to stand out even when the mound itself seems to dissolve into the surrounding ground.