A fulacht fia, as these sites are known, is a Bronze Age outdoor cooking place, typically consisting of a water-filled trough, a hearth for heating stones, and a mound of cracked, fire-shattered rock that accumulated over years of use. Stones were heated in the fire and dropped into the trough to bring the water to a boil, a surprisingly efficient method that archaeologists have tested and confirmed works well for cooking meat. What makes the Derryfadda example notable is the sheer scale of activity it represents, and the unusual circumstances of its preservation.

The site comprises a timber-lined trough measuring roughly 2.4 metres by 1.75 metres and 0.6 metres deep, alongside a hearth area approximately 1.2 metres long and 0.8 metres wide, and a substantial mound of burnt material spreading some 14 metres north to south and 19 metres east to west. That mound is the accumulated debris of repeated firings, and when excavators Murray and Ó Néill assessed its volume in 1999, they estimated that the site had been used somewhere between 400 and 500 times over its working life. That figure transforms what might seem like a simple prehistoric feature into something closer to a regularly visited, heavily used installation, perhaps serving a wider community across many generations. Adding a further layer of interest, a togher, a timber trackway laid across boggy ground to allow passage through otherwise difficult terrain, was found overlying the site, suggesting the landscape around Derryfadda continued to attract human activity long after the fulacht fia itself had fallen out of use.