It measures roughly 12.5 metres from north to south and 9.6 metres east to west, rising only about half a metre from the ground. That modest silhouette, however, is the accumulated debris of repeated prehistoric cooking, the kind of feature known in Irish archaeology as a fulacht fia.

A fulacht fia, sometimes called a burnt mound, is one of the most common prehistoric monument types found across Ireland. The basic principle involves heating stones in a fire, dropping them into a water-filled trough to bring it to a boil, and using that boiling water to cook food, though some researchers have proposed additional uses including textile processing or bathing. The cracked, heat-shattered stones that resulted from this repeated heating and cooling were raked out and discarded nearby, and over centuries of use those discarded stones built up into the low, horseshoe- or D-shaped mounds that archaeologists now recognise across the Irish countryside. Most date to the Bronze Age, roughly 1500 to 500 BC, though some examples fall outside that range. The southern end of the Ardglass example has accumulated dumped earth and vegetation over time, which partly obscures its original profile.