It is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, typically Bronze Age in date, and consisting of a trough dug near a water source and a surrounding mound built up from the debris of repeated use. The mechanics were straightforward: stones were heated in a fire, dropped into a water-filled trough to bring it to a boil, and used to cook meat. Over time, the stones cracked and shattered from the thermal stress, and the discarded fragments accumulated into the horseshoe-shaped mound that survives.

The mound at Rossmackowen measures 8.5 metres north to south and 5.4 metres east to west, rising to a modest height of 0.85 metres. Its opening, about 2.2 metres wide, faces west, which is where the trough would originally have lain, close to the stream on the eastern bank. The mound is composed of heat-shattered stones and charcoal-enriched soil, and burnt material remains visible particularly along its south-western side. Gorse and heather have colonised much of the surface, and erosion has worn down the northern and southern ends, giving the site a slightly softened outline that can make it easy to overlook if you do not know what to look for.