It rises only about half a metre from the surrounding ground, the kind of feature that a casual walker might take for a natural undulation. In fact it is a fulacht fia, one of the most common prehistoric monument types found across Ireland, and one that was already partially levelled by around 1974, reducing what may once have been a more prominent earthwork to its present subdued profile.

Fulachtaí fia, sometimes called burnt mounds, are generally understood to be the remains of ancient cooking or hot water sites, most commonly dated to the Bronze Age, roughly 1500 to 500 BC. They typically consist of a horseshoe-shaped or circular mound of fire-cracked stone and charcoal, accumulated over repeated episodes of use. The usual interpretation is that stones were heated in a fire and then dropped into a water-filled trough, bringing the water rapidly to the boil. The shattered, heat-fractured stones, no longer useful, were piled to the side, and it is this accumulation that forms the visible mound. The example at Garranbeg fits this general pattern, though its levelling around 1974, almost certainly the result of agricultural work, means that much of what might have been visible or recoverable above ground has been disturbed or removed.