The term refers to burnt mounds, typically crescent or horseshoe-shaped spreads of fire-cracked stone and dark, charred soil, thought to represent ancient cooking or industrial sites where stones were heated in a fire and dropped into a water-filled trough to bring it to the boil. The example at Derryville in County Tipperary adds a small but telling variation to this familiar pattern: it came to light not through deliberate investigation but as a consequence of construction work, emerging from beneath a bulldozed field on the edge of a peat-fringed slope.

In 2002, archaeologist Richard Crumlish was conducting monitoring work on a development site lying to the south of a known earthwork when the ground gave up something older and less expected. Alongside the fulacht fiadh itself, two burnt spreads and two charcoal spreads were identified. A subsequent excavation revealed the full extent of the site, which measured approximately 20 metres by 14 metres and sat on a west-facing slope beside an area of peat, the kind of boggy, waterlogged ground that would have made such a site practical in prehistory, providing a ready water source for the trough. The topsoil stripping by machinery had disrupted the mound enough to obscure its original shape, which might otherwise have conformed to the classic curved outline. What remained nonetheless included two pits and a rock-cut trough, the latter hewn directly into the bedrock rather than constructed from timber or other materials, a detail that speaks to the particular geology of the spot and the effort invested by whoever used it.