A fulacht fia is, broadly speaking, an ancient cooking or processing site, typically identified by a mound of fire-cracked stones and charcoal left behind after repeated use of a water trough heated by dropping in stones from a fire. They are found in their thousands across Ireland, usually near water, and this one is no exception, sitting on the western side of a stream that still runs along the castle's edge.

The site came to light not through planned excavation but by accident, during the monitoring of a sewage trench dug in 1989. The burnt material was observed spreading across an area of roughly 16 metres north to south and 12 metres east to west, which is a substantial footprint for a monument of this type. That the burnt layer has been incorporated into a field fence running north from the bawn corner suggests the material had long since been absorbed into the working fabric of the landscape without anyone fully realising what it was. Further investigation at the time was not possible, so the full extent and character of the deposit remains unknown. What is clear is that this prehistoric activity predates the castle entirely; Barryscourt, a tower house associated with the Roche family, belongs to the medieval period, while fulachtaí fia are generally Bronze Age features, meaning the ground here had already seen thousands of years of human use before any stone wall was ever raised above it.