It measures fifteen metres long, seventeen metres wide, and rises about a metre from the surrounding ground, with an opening six metres across facing northwest. The dark, crumbly material it is made from is the giveaway: this is a fulacht fia, a type of prehistoric cooking or processing site found in enormous numbers across Ireland, and particularly concentrated in Cork and Kerry.

Fulachtaí fia, the plural form, are among the most common archaeological monuments in the Irish countryside, yet they remain somewhat puzzling. The basic principle is well understood: a trough, usually timber-lined, was filled with water and heated by dropping fire-cracked stones into it; those stones, once spent and discarded, accumulated over generations into the characteristic horseshoe mound, with the open end of the horseshoe typically marking where the trough once sat. What the sites were actually used for, whether cooking, textile processing, bathing, or brewing, is still debated. The location of the Derryroe example is entirely typical: wet, low-lying ground near running water, which would have made filling and refilling a trough relatively straightforward. The burnt and shattered stone that forms the mound is the lasting physical trace of repeated, sustained use, probably across the Bronze Age, roughly 2000 to 500 BC.