It is, in fact, a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish landscape. These horseshoe-shaped mounds, found in their thousands across Ireland, are the accumulated debris of repeated fire and water: stones were heated in a fire, dropped into a water-filled trough to bring it rapidly to the boil, then discarded when they cracked from the thermal shock. Over time, the broken, charcoal-stained fragments built up into the characteristic mound shape that survives today.

The monument at An Inse Mhór measures roughly seven metres east to west and just over six metres north to south, rising to about a metre in height. Its opening, roughly 1.7 metres wide, faces west, which is typical of the form. The dark, charcoal-enriched soil mixed through the shattered stone gives some sense of the repeated burning that took place here, though exactly what the site was used for remains debated. Cooking meat is the most widely accepted explanation, though brewing, hide-working, and bathing have all been proposed by archaeologists over the years. The southern edge of the mound has suffered some damage from recent drainage works, a common fate for low earthwork monuments that sit in working farmland.