It is a fulacht fia, a type of Bronze Age cooking site found in great numbers across Ireland, typically identified by the distinctive horseshoe-shaped mound of heat-shattered stones that accumulates around a central trough. The usual interpretation is that stones were heated in a fire, then dropped into a water-filled pit to bring the contents to a boil; the cracked and blackened stones, no longer useful, were simply tossed aside over time, building up the mound that survives today. What makes this one quietly remarkable is not any single feature but the simple fact of its survival, largely intact, in a landscape that has continued to be farmed around it for millennia.

The mound at Rossnashunsoge measures roughly 10.5 metres north to south and 9.5 metres east to west, rising to about 1.1 metres in height. Its opening, some 3.5 metres wide, faces west. The mound material is the characteristic mix of heat-shattered stone and charcoal-enriched soil that archaeologists use to identify these sites. A wire fence now crosses the northern arm of the horseshoe, and the whole structure sits within a network of field boundaries in an area of rough grazing. Approximately 15 metres to the north-east lies a second fulacht fia, a proximity that is not unusual; these sites frequently cluster together, suggesting repeated or communal use of particularly suitable spots over long periods. Wet hollows like this one would have provided a ready water supply, which was essential to the whole process.