At eighteen metres long and only twenty centimetres high, it would pass entirely unnoticed were it not for the plough, which periodically turns up its true contents: burnt and broken stones, the hallmark signature of a fulacht fiadh, or burnt mound. These are among the most common archaeological monument types in Ireland, yet their precise purpose remains genuinely debated. The most widely accepted interpretation is that they served as outdoor cooking sites, where stones were heated in a fire, dropped into a water-filled trough to bring it to the boil, and then discarded in a crescent or kidney-shaped heap once they cracked and became useless. The distinctive shape of the mound at Coolboy is entirely consistent with this pattern.

The mound sits on the north bank of a small east-west stream, which itself feeds into the Kayle stream roughly 350 metres to the west. Water proximity is almost a prerequisite for this type of site, and the stream here would have provided exactly what the process required. What makes this particular location quietly interesting is its relationship to a neighbouring feature: the remains of a horizontal mill lie just ten metres to the south-east, positioned at a slight bend in the same stream. A horizontal mill, sometimes called a Norse mill, is an early medieval milling technology in which a wheel mounted flat beneath the millstone is driven directly by the flow of water beneath it, without the gearing required by a vertical mill. The two monuments are distinct in date and function, yet both drew on the same modest watercourse, suggesting the stream was a persistent focus of activity across different periods.