This is a fulacht fia, a type of prehistoric cooking site found in great numbers across Ireland, typically consisting of a burnt mound of heat-shattered stone built up around a water trough. The trough would have been filled from a nearby water source, stones heated in a fire and dropped in to bring the water to a boil, and the cracked, spent stones piled to either side until the characteristic crescent or horseshoe shape developed. What makes the example at Acres quietly compelling is how legible it remains: the mound measures 14.6 metres north to south and 13.1 metres east to west, rises to a maximum height of 1.5 metres at its northern edge, and retains a well-defined central depression between 0.2 and 0.8 metres deep, essentially the ghost of the trough itself.

The site sits on a level terrace just fifteen metres wide, on gently sloping ground where the rock breaks the surface. A short bank of around one metre extends from the north-eastern side of the mound, and roughly three metres to the east a spring emerges from the base of a low rock bluff, one to one and a half metres high. That spring is not incidental. Fulachtaí fia are almost invariably found close to reliable water sources, and here the relationship between mound and spring is unusually immediate and easy to read. The site was noted on Tim Robinson's 1977 map of the Burren, which means it was already a recognised landmark of the landscape well before any formal survey work began.