It measures eleven metres long, ten metres wide, and rises to just over a metre in height, with a four-metre opening facing north-east. That distinctive shape is the calling card of a fulacht fia, a type of prehistoric cooking site found in large numbers across Ireland, particularly in low-lying, waterlogged ground near streams or wells. The working theory is that stones were heated in a fire, then dropped into a water-filled trough to bring it to boiling point, the cracked and shattered remains of those stones gradually accumulating into the characteristic mound that survives today. What is less usual about Garrynagearagh is that a second fulacht fia lies immediately to the south, the two sites sitting adjacent to one another in the same field.

Fulachtaí fia are among the most common archaeological monument types in Ireland, with thousands recorded nationwide, yet individual examples rarely attract much attention. Their precise function has been debated for decades. Cooking is the dominant explanation, supported by experiments showing the heated-stone method works efficiently for boiling large quantities of meat, but proposals have also included bathing, textile processing, and brewing. The pairing of two such sites in close proximity at Garrynagearagh adds a quiet layer of interest; whether they were in use simultaneously, or represent activity at different periods in the same favoured spot beside the well, is not something the visible archaeology can answer on its own.