It is a burnt mound, known in Irish archaeology by the term fulacht fiadh, and these curious features are among the most numerous prehistoric monuments in Ireland. The basic idea is straightforward: stones were heated in a fire and then dropped into a water-filled trough, bringing the water rapidly to the boil. The cracked, heat-shattered stones were discarded to the side, and over repeated use they accumulated into the characteristic horseshoe or kidney-shaped mounds that survive today. What they were actually used for, whether cooking, bathing, textile processing, or some combination of purposes, remains a matter of ongoing debate among archaeologists.

Burnt mounds are typically dated to the Bronze Age, roughly between 2000 and 500 BC, and they tend to cluster near reliable water sources, streams, springs, or boggy ground. Mayo has a considerable number of them, a reflection both of the county's prehistoric settlement and of the wet, peaty conditions that have helped preserve organic material and earthworks across the landscape for millennia. The Ballygarriff example is one of many such sites recorded across the county, each representing repeated, organised activity by communities who understood how to exploit local geology and hydrology for their practical needs.