It is a fulacht fia, a type of ancient cooking or processing site found in great numbers across Ireland, and this one has been worn by time and agriculture to the point where it is, in the words of those who recorded it, barely perceptible.

Fulachtaí fia typically consist of a horseshoe-shaped mound of fire-cracked stone and charred material, the accumulated debris from a process in which stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil. The trough itself was usually cut into the ground and lined with wood or stone. The method works efficiently, and experimental archaeology has shown that large volumes of water can be brought to a full boil surprisingly quickly. These sites cluster near streams and wet ground almost without exception, and the Mountrivers example follows that pattern precisely, sitting to the west of a watercourse that would have supplied the water essential to whatever was happening here. Whether the site was used for cooking meat, processing hides, or some other purpose is a question that continues to occupy archaeologists. The burnt mound at Mountrivers has not, on current evidence, yielded answers specific to this location, but its placement and composition fit the wider pattern well.