The term refers to a prehistoric cooking site, typically identified by a mound of fire-cracked stones that accumulated over repeated use. The method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil, a process that works with surprising efficiency but leaves behind a distinctive scatter of shattered, heat-reddened rock.

What places the Garryndruig example in context is the combination of its topography and its immediate environment. The site sits on ground that slopes down toward wet land to the west, which is entirely characteristic of the type. Fulachtaí fia are almost invariably found close to a reliable water source, whether a stream, a spring, or naturally boggy ground, since a constant supply of water was central to how they functioned. The north-facing slope would have kept the area cool and damp, the kind of marginal, wet ground that Bronze Age communities seemed to favour for this activity, perhaps precisely because it was unsuitable for cultivation or settlement. The visible evidence here is a spread of the burnt and broken stone rather than a preserved mound, which suggests the site has been disturbed by ploughing over a long period.