Beneath the grass, however, lies a dense layer of fire-cracked stone and charcoal fragments packed into grey-black soil, the accumulated debris of repeated, deliberate burning carried out thousands of years ago. This is a burnt mound, a type of prehistoric site found widely across Ireland and Britain, and thought to represent a place where water was heated by dropping fire-heated stones into a trough or pit. The exact purpose remains debated: cooking, bathing, and industrial processes such as working leather have all been proposed, but the sheer quantity of discarded burnt stone at such sites points to sustained, organised activity over long periods.

The mound at Inishcoe came to light in 1996, when ploughing at the edge of a field clipped its north-east side and exposed what lay beneath. At that point it measured roughly eight metres from north-west to south-east and about five metres across, though a field drain had already cut into and truncated the south-west end. In the exposed drain section, the characteristic burnt-stone layer could be read clearly, running to a depth of around 0.4 metres. The damage from both the plough and the drain is a reminder of how fragile such low-profile sites are in an agricultural landscape; without the accident of disturbance, this one might have remained unnoticed indefinitely. About 150 metres to the south-south-east lies a rath, a circular earthwork enclosure of the early medieval period typically used as a farmstead, which suggests that this corner of Mayo has seen sustained human occupation across very different eras.