They are, in essence, the remains of ancient cooking sites: a trough would be filled with water, stones heated in a nearby fire and dropped in to bring the water to a boil, and meat cooked in the resulting heat. The discarded, shattered stones and charcoal-rich soil accumulated over time into a low mound, and it is that mound, rather than any visible structure, that survives. The example at Larkhill in County Sligo sits in marshy ground beside a small pool, exactly the kind of wet, low-lying setting that seems to have been preferred for this type of site, since a ready water source was essential to the whole process.

The mound itself is oval, measuring roughly nine metres along its longer axis and seven metres across, rising to about 0.8 metres above the surrounding ground. It is composed of burnt stones and dark, charcoal-flecked soil, the accumulated debris of repeated use. What makes the Larkhill site particularly interesting is its immediate neighbourhood: two further possible fulachtaí fia lie approximately forty metres to the west, suggesting that this stretch of marshy ground was used over and over, perhaps across generations, as a site of communal food preparation or some other activity involving sustained, controlled heating of water.