What the overgrowth reveals, if you look carefully at its outline, is a horseshoe form, open at the south-south-west, roughly eleven metres from north to south and nine metres across, rising less than a metre above the surrounding wetland. That modest profile conceals a considerable age and a particular kind of prehistoric industry.

The mound is a fulacht fia, a type of site found in great numbers across Ireland, particularly in low-lying or waterlogged ground. The term refers to the accumulated debris left behind by a repeated process: stones were heated in a fire, then dropped into a water-filled trough to bring it rapidly to the boil. Over time, the used and shattered stones were raked out and piled up, forming the distinctive mound that survives today. Thousands of these sites are known across the country, most dating to the Bronze Age, and their concentration near wet ground is no accident, since a reliable water source was essential to the whole operation. What exactly the boiling water was used for remains debated; cooking is the most commonly cited explanation, but brewing, textile processing, and bathing have all been proposed. The horseshoe shape of the mound at Horsemount, with its open southern end, is typical, reflecting the way material was thrown aside from a central working area over repeated use.