It measures roughly thirteen metres north to south and eight metres east to west, and at its highest point reaches only thirty centimetres. Beneath the grass lies a spread of burnt and broken stone, the characteristic signature of a fulacht fia, an ancient cooking site where water was heated by dropping fire-cracked stones into a trough. The mound at Rinnamona is unassuming to the point of near-invisibility, which makes it easy to overlook just how much ancient activity this particular patch of ground seems to have concentrated.

Fulachtaí fia are among the most common prehistoric monument types in Ireland, typically associated with the Bronze Age, and they are almost always found near water. The turlough here, a seasonal lake that floods in winter and drains in summer through the underlying limestone, would have provided a reliable water source, which likely explains why this spot attracted repeated use. At the eastern edge of the mound there are two grassed-over quarry-holes, each around two and a half metres in diameter, and a larger one to the northwest measuring roughly four by three metres; these hollows are thought to represent the troughs into which heated stones were cast. A second fulacht fia lies approximately twelve metres to the northwest. What makes Rinnamona slightly unusual within this already-clustered landscape is a question of cartographic confusion: three further fulachtaí fia were at one point recorded about a hundred and fifty metres to the north, but it appears that this information was corrupted during the transfer between map scales. One of those putative sites may in fact be a gravel bank, roughly nine metres long and three metres wide, that sits thirty metres north of the second monument and was likely misread as something it is not. The archaeology here is real, but the paper record around it has required untangling.