These low, horseshoe-shaped mounds, typically dark with charred and shattered stone, are among the most common prehistoric monuments on the island, and also among the most debated. The general picture is reasonably well established: a trough was dug into the ground, lined with wood or stone, and filled with water; stones were heated in a nearby fire and dropped into the trough to bring the water to a boil. What that boiling water was actually used for, whether cooking, bathing, textile processing, or something else entirely, remains a genuinely open question in Irish archaeology.

The Cashel example belongs to a tradition that spans the Bronze Age in particular, roughly 2000 to 500 BC, though some sites have yielded dates stretching earlier or later. The characteristic burnt mound that gives these sites their dark, humped appearance is formed from the discarded cracked stones, which fracture when repeatedly heated and plunged into cold water and become useless for further heating. Over centuries, the spoil accumulates into the low crescentic shape that field surveyors learn to recognise from a distance. Mayo, with its boggy, water-retentive soils, preserves these features well; the wet ground conditions that made fulachta fia practical in the first place also helped seal them from later disturbance.