It measures roughly eleven metres long, eleven metres wide, and just half a metre high, and to the casual eye it might pass for nothing more than a slight rise in soggy ground. It is, in fact, the remains of a fulacht fia, one of the most common yet persistently puzzling monument types in the Irish archaeological landscape.

Fulachtaí fia (the plural form) are prehistoric cooking sites, found in their thousands across Ireland, typically dating from the Bronze Age though some examples span into later periods. The standard interpretation is that they were used for boiling water: a trough, usually timber-lined or stone-lined and dug into the ground, was filled with water, and stones heated in a nearby fire were dropped in to bring the water to temperature. Over time, the cracked and heat-shattered stones were raked out and discarded, accumulating into the characteristic horseshoe-shaped or irregular mounds that survive today. The burnt, fire-cracked stone that makes up these mounds gives them their distinctive dark, friable appearance when cut into. Why they so consistently appear in wet, low-lying ground is partly practical: proximity to a water source was essential to their operation, and marshy areas provided that readily. The site at Derry fits this pattern precisely, its overgrown mound sitting in exactly the kind of damp, marginal ground where these monuments are most often encountered.