The mound measures ten metres in length, stands only around sixty centimetres high, and is composed almost entirely of burnt and fire-cracked stone. This is a fulacht fia, a type of cooking or heating site found in enormous numbers across Ireland, and one of the more persistently debated categories of prehistoric monument the island has to offer.

Fulachtaí fia, the plural form, are typically Bronze Age features, though some examples have earlier or later origins. The standard interpretation is that they functioned as outdoor cooking sites: a trough dug into the ground would be filled with water, and stones heated in a nearby fire would be dropped in to bring the water to a boil, sufficient to cook meat wrapped in straw or placed in a container. The stones, cracked and shattered by repeated heating and sudden cooling, were then discarded in a heap beside the trough. Over centuries of use, or repeated episodes of use, these discarded stones accumulated into the characteristic horseshoe shape, with the open end of the curve typically corresponding to where the trough once sat. The marshy, low-lying ground at Clonmoyle is exactly the kind of location where these sites tend to cluster, since a ready and reliable water source was essential to the whole process. Some researchers have also proposed that fulachtaí fia served functions beyond cooking, including hide processing or even communal bathing, though none of these alternatives has displaced the cooking explanation entirely.