It is a fulacht fia, a type of monument found across Ireland in the thousands, typically Bronze Age in date, and consisting of a mound of fire-cracked stone and charcoal left over from repeated episodes of water-heating. The usual interpretation is that stones were heated in a fire and dropped into a water-filled trough until the water boiled, the cracked and spent stones then being raked aside to form the characteristic horseshoe or kidney-shaped mound. The site at Lackan conforms to this general picture, though its waterlogged, poorly drained setting is entirely typical; fulachtaí fia are almost always found close to water or in marshy ground.

When the site was first recorded in 1993, it appeared as a low, irregular mound roughly ten metres north to south and nine metres east to west, composed of heat-shattered stone and charcoal. The northern end was noticeably higher and carried a small semicircular depression about 0.6 metres wide, possibly a remnant of the original trough. A raised scarp half a metre high defined the southern edge, while the western and eastern margins were harder to trace. By the time of a follow-up inspection in 2013, land improvement works had graded the mound down somewhat. It now presents as a low, roughly circular rise, slightly larger in footprint at around twelve metres by eleven, but no taller than 0.5 to 0.6 metres at its highest northern point. The southern scarp has softened, and much of the mound's edge now grades almost imperceptibly into the field around it, the kind of slow erasure that happens when ancient earthworks sit in agricultural land long enough.