This is a fulacht fia, a type of monument found in great numbers across Ireland, typically dating from the Bronze Age. The characteristic form is a horseshoe-shaped or roughly circular mound of fire-cracked stone, built up over many uses, surrounding a trough that would once have been lined with timber or stone and filled with water. Stones heated in a fire were dropped into the trough to bring the water to a boil, and meat was cooked in the resulting heat. The method is surprisingly efficient, and experiments have confirmed that a trough of this size could boil water within half an hour.

The example at Coolderry sits in low-lying esker terrain, eskers being the long, winding ridges of gravel and sand deposited by meltwater streams flowing beneath glaciers at the end of the last Ice Age. This kind of landscape is characteristic of the Irish midlands, and its free-draining gravels would have made it attractive for settlement and activity over long periods. The mound here measures roughly 19.3 metres north to south and 17.7 metres east to west, rising to about 0.9 metres in height. It carries two depressions: a central one measuring approximately 4.1 by 3.9 metres and reaching a depth of between 0.5 and 0.82 metres, and a second, more elongated depression at the southern end, measuring 4.4 by 1.7 metres at a shallower depth. A river runs roughly 100 metres to the south, which is entirely typical. Fulachtaí fia are almost always found close to a water source, proximity to which was essential to the whole process.