This is a fulacht fia, a type of prehistoric cooking site found in considerable numbers across Ireland, typically identified by the distinctive horseshoe or mound shape left behind by thousands of fire-cracked stones discarded after repeated heating and quenching. The mound at Castlewrixon measures just over ten metres on its longer axis and rises to about three quarters of a metre, modest dimensions that are nonetheless entirely typical of the form.

The standard interpretation of a fulacht fia is that water, drawn from a nearby source, was heated by dropping fire-heated stones into a trough or pit, allowing food to be boiled or meat to be cooked over extended periods. The proximity of this example to the stream fits the pattern precisely; access to water was a practical necessity of the process, and the stones, once spent and cracked, were piled to the side, gradually accumulating into the low mounds archaeologists find today. Most fulachtaí fia in Ireland date to the Bronze Age, though some sites show evidence of use across a longer span. A waterlogged area persists at the south-eastern edge of this particular mound, a small and telling detail that hints at the wet conditions that would originally have made the site useful and that have, in turn, helped to preserve it.