A fulacht fia, to use the Irish term, is a type of burnt mound found in great numbers across Ireland, typically dating to the Bronze Age. The principle was straightforward: stones were heated in a fire, then dropped into a water-filled trough, bringing it rapidly to a boil. Repeated heating and quenching cracked the stones into fragments, and it is the accumulated debris of those broken, fire-reddened pieces that forms the distinctive horseshoe shape still visible at the surface today.

The Coskeam example is a well-preserved specimen of the type. Its burnt mound measures roughly 16.6 metres north to south and 10.3 metres east to west, rising to a maximum height of nearly two metres. The horseshoe opens to the west, which is precisely where a small spring emerges from the base of a low limestone bluff, providing the reliable water source that would have made this spot worth returning to. Within the trough area, two pairs of earthfast stone slabs have been identified. The first pair, set perpendicularly and lying just south-east of the spring, may represent the remains of a stone-lined trough; the second pair, standing roughly a metre apart at the northern interior of the mound, could be the remnants of a second. A shallow depression in the eastern part of the mound adds a further detail to the site's internal arrangement. The valley itself, enclosed on three sides by low cliffs and bluffs with rushy, wet ground underfoot, is exactly the kind of sheltered, well-watered hollow in which these sites were habitually established.