Measuring roughly 19 metres north to south and 14 metres east to west, it is composed of heat-shattered stones and charcoal-enriched soil, the characteristic signature of a fulacht fia. These are among the most common prehistoric monument types in Ireland, generally interpreted as Bronze Age cooking sites, where stones were heated in fire and dropped into a water-filled trough to bring the contents to a boil. The distinctive horseshoe-shaped spreads of burnt, cracked stone are the remnants of that repeated heating and discard, accumulated over what may have been many episodes of use.

What makes the Farranastig example particularly interesting is a detail that came to light not through formal excavation but through ordinary agricultural work. When a fence bordering a lane on the western edge of the site was removed, the remains of a wooden trough were uncovered beneath it. Wooden troughs are the perishable component that rarely survives, and their discovery tends to confirm the cooking or processing function of a site in a way that the stone spread alone cannot. The trough here had been preserved, most likely by waterlogged or anaerobic soil conditions, until that boundary fence was lifted and exposed it. No formal excavation appears to have followed, and the detail survives only as local information passed on rather than as a measured archaeological record.