Beneath the disturbed material, covered by six or eight large stones, was a hollow in the ground, most likely the trough that gave the whole structure its purpose.

A fulacht fia is a type of prehistoric cooking site found in enormous numbers across Ireland, typically beside water and in low-lying, damp ground. The standard interpretation is that stones were heated in a fire and then dropped into a water-filled trough, bringing the water to a boil for cooking meat. The site at Reacaslagh fits the pattern closely. When the Castleisland District Archaeological Survey recorded it in 1987, the monument had already been levelled by the landowner, but its signature was still legible: a roughly horseshoe-shaped spread of burnt black earth and heat-shattered red stone, measuring about 10.7 metres on the longer axis, sitting in a corner of wet pasture with the Clydagh River forming the field boundary to the northwest. The landowner recalled the mound as it had stood, somewhere between one and a half and nearly two metres high, arranged in that characteristic curved form. The horseshoe shape is typical of fulachtaí fia, thought to result from the gradual accumulation of spent, cracked stone piled around three sides of the trough over repeated use.

What makes the Reacaslagh example quietly telling is how much information survived the levelling. The spread of scorched material remained visible in the field, the landowner's memory preserved the original height and outline, and the capped hollow beneath the stones pointed directly to where the water would have been heated. The site had been erased as a physical monument but not quite as a record.