It measures just under thirteen metres in length, a little over ten metres wide, and rises to about a metre in height, with an opening of nearly four metres facing west. The material that makes up the mound is burnt, the accumulated residue of repeated heating, and that detail is the key to understanding what this place once was.

This is a fulacht fia, a type of prehistoric cooking or processing site found in great numbers across Ireland, typically dating from the Bronze Age, roughly 2000 to 500 BC. The basic principle involves heating stones in a fire and then dropping them into a water-filled trough to bring the water to boiling point. The cracked and heat-shattered stones are then discarded to the side, and over many uses they accumulate into exactly the kind of horseshoe-shaped mound seen here, with the open end corresponding to where the trough would have been. Whether these sites were used primarily for cooking meat, for preparing hides, or for bathing and other communal purposes is still debated among archaeologists, and it is likely that different sites served different needs. What makes the Horsemountmountain example quietly notable is that it is not alone: a second fulacht fia lies approximately fifty metres to the south, suggesting that this low, wet ground was revisited or used concurrently by people who found something about this particular spot worth returning to.