A fulacht fia is a type of monument found in great numbers across Ireland, typically consisting of a mound of fire-cracked stone accumulated beside a water source, the debris of repeated episodes of heating stones and dropping them into a trough of water to bring it to the boil. The mound at Ballyhimmin sits at the base of a south-east-facing slope in a flat river valley, with a stream running nearby to the south, and a moated site, a type of enclosed medieval farmstead defined by a broad ditch and bank, just forty metres to the west. The two monuments are unrelated in date, but their proximity is a reminder of how repeatedly attractive certain low-lying, well-watered ground proved across different eras.

The site only came to attention in 1955, when ploughing disturbed a spread of burnt material and prompted excavation. At that point it was still recognisable as a low horseshoe-shaped mound, roughly ten to eleven metres across and about three-quarters of a metre high, its straight edge once defined by a stream that has since dried up. The excavation, later described by Prendergast in 1977, uncovered a rectangular wooden trough in the western part of the mound, set into a pit dug down into the underlying marl. The trough measured approximately 1.4 metres long by 1.5 metres wide, its base laid with five carefully trimmed planks, two of oak and three of alder, fitted closely together. Only the lower portions of two side planks survived, both alder, and the whole structure rested on a bed of willow and alder branches. Traces of moss around the wood suggested the trough may have been caulked to make it watertight. The mound itself was built up from burnt stone described as flaggy quartzose sandstone mixed with ash, and charcoal samples showed that alder, ash, hazel, willow, poplar, and holly had all served as fuel at various times.

Today very little of this survives above ground. The horseshoe mound has reduced to a slight rise, and the stream that once defined its edge is gone. The detail that stays with you is in the carpentry: planks trimmed flat and fitted with evident care, a modest but deliberate piece of construction sunk into boggy ground, used, and eventually buried under its own accumulated waste.