This is a fulacht fia, a type of ancient outdoor cooking site found in extraordinary numbers throughout the island, particularly in boggy or waterlogged ground. The typical arrangement involves a trough dug into the earth, often lined with wood or stone, which was filled with water and then heated by dropping fire-cracked rocks into it. The mound itself is the accumulated debris of that process, the discarded stones split by repeated heating and quenching, building up over generations of use into the horseshoe-shaped heaps that field archaeologists still recognise today.

Fulachtaí fia, to use the Irish plural, are among the most common archaeological monuments in Ireland, with thousands recorded nationwide. Most date to the Bronze Age, roughly between 1500 and 500 BC, though some sites show evidence of use stretching into the Iron Age. The sheer number of these sites has long puzzled researchers. Cooking is the most widely accepted explanation, but experiments and scholarly debate have also raised the possibilities of textile processing, bathing, or even brewing. The Cashel example in Mayo represents one of countless such sites distributed across the western counties, where the damp, peaty terrain both preserved the organic remains and provided the ready water supply the process required. Mayo itself has a particularly dense concentration of prehistoric monuments, reflecting long and sustained human activity in the region from the Neolithic period onwards.