The discolouration marks what survives of a fulacht fia, a type of prehistoric cooking site found widely across Ireland, typically consisting of a timber or stone trough filled with water, heated by dropping fire-cracked stones into it until the water boiled. The stones, once used, were discarded into a mound beside the trough. It is those same fire-shattered, heat-blackened stones, broken down and spread across the field over centuries of disturbance, that produce the characteristic dark signature visible at Glanshearoon.

The site was recorded in 1986 during the Castleisland District Archaeological Survey. By that point it had already been levelled, its original mound reduced to a slight rise barely perceptible above the surrounding field surface. Surveyors noted that the spread of burnt material was unmistakable as a black stain against the normal soil colour, but that no trace of the trough itself could be identified. Fulachta fia are among the most common prehistoric monuments in Ireland, dating mostly to the Bronze Age, and they tend to survive best in low-lying, damp ground. A ploughed field is one of the harder environments for such a site to endure, and at Glanshearoon the agricultural activity had clearly taken its toll, erasing the raised mound that would once have signalled the site's presence from a distance.