What survives today is a roughly circular mound of fire-cracked material, measuring about 14.6 metres east to west and rising only a modest 0.25 metres above the surrounding ground. At its centre sits a shallow depression, roughly 6 metres across in one direction and 3 metres in the other, scooped about half a metre deep. A tail of the same material extends to the northwest, adding another stretch of burnt debris to the overall shape.

This is a fulacht fia, a type of prehistoric cooking or processing site found in enormous numbers across Ireland. The basic principle involved heating stones in a fire, dropping them into a water-filled trough to bring it to the boil, and using that hot water for cooking, possibly for brewing, or perhaps for industrial processes such as working leather or wool. Over time, the shattered, heat-spent stones were raked aside into the characteristic horseshoe or oval mound that survives at Derrygalun. The central depression is typically all that remains of the original trough. Thousands of these sites are known across the country, most dating to the Bronze Age, yet each one represents a specific act of repeated, practical use, somebody returning to this spot, building the fire, and heating the water, over and over. The Derrygalun example was once more substantial than it appears now. Local memory holds that the mound was partially levelled around 1955, reducing its height from whatever it formerly stood at to the shallow profile visible today.