The structure at Carhoomeengar is a fulacht fia, a term used in Irish archaeology to describe the burnt mounds left behind by an ancient method of cooking or heating water. The principle was straightforward: stones were heated in a fire, then dropped into a water-filled trough until the water boiled. The shattered, heat-cracked stone was raked out and piled up around the trough, and it is precisely this accumulation of scorched, fragmented material that survives as a mound. Fulachtaí fia are among the most common prehistoric monuments in Ireland, found in their thousands across the country, almost always near water.

What survives at Carhoomeengar is the eroded remnant of what was once a single horseshoe-shaped mound, roughly twelve metres north to south and ten metres east to west. Erosion on the eastern side has split it into two distinct crescent-shaped sections, one to the north and one to the south, each no more than half a metre high at their tallest points. Both are composed of burnt material and sit embedded in the surrounding bog, which has, in a sense, preserved them by keeping the ground around them waterlogged and undisturbed. The opening of the original horseshoe, about three and a half metres wide, faces west. A stream runs approximately seventy metres to the north-west, almost certainly the same water source that whoever used this site would have relied upon.