It is made almost entirely of burnt stone and charcoal-blackened earth, the accumulated debris of what archaeologists call a fulacht fia, and it has been there, largely undisturbed, for somewhere between three and four thousand years.

Fulachtaí fia are among the most common prehistoric monuments in Ireland, found in their thousands across the country, almost always near water and in low-lying or boggy ground. The basic principle, as understood from both excavation and experiment, is that stones were heated in a fire and then dropped into a water-filled trough to bring the water to a boil. What the troughs were used for is still debated; cooking meat is the most widely accepted explanation, though brewing, hide-working, and bathing have all been proposed. The spent, cracked stones were discarded to the side, and over repeated use they built up into the characteristic mounds we see today. The example at Horsemount fits the type closely, positioned to the south of a well and on the eastern bank of a stream, exactly the kind of wet, water-adjacent setting these sites favour. What makes its location particularly notable is the density of monuments nearby: two further fulachtaí fia lie within sixty metres to the south-east, suggesting this stretch of ground saw repeated, possibly sustained, use over a long period.