They appear as low, horseshoe-shaped mounds, typically found close to water, and date mostly to the Bronze Age, somewhere between 1500 and 500 BC. The working theory, long debated, is that they were used for cooking: stones were heated in a fire, dropped into a water-filled trough, and brought to the boil. The mound itself is the accumulated debris of fire-cracked stone, discarded after each use because the thermal shock renders the rock useless for reheating. The one at Knocknalappa, in County Clare, is a quiet example of this widespread but still puzzling tradition.

Knocknalappa sits in a part of Clare that would have seen considerable Bronze Age activity, and a fulacht fia in this location fits a familiar pattern: low-lying ground, likely with a natural water source nearby, and proximity to other traces of prehistoric settlement. The site's name offers its own layer of interest. "Knocknalappa" derives from the Irish, with "cnoc" meaning hill, though the second element is less straightforward to unpick. The fulacht itself would originally have been unobtrusive, and in many cases these sites survive precisely because they were too unremarkable to disturb, left alone by later farmers who may have taken them for natural rises in the ground.