This is a fulacht fia, a type of monument found in great numbers across Ireland, believed to have functioned as a cooking site, though possibly also used for textile processing or bathing. The characteristic method involved heating stones in a fire, dropping them into a water-filled trough to bring it to boiling point, and repeating the process until food was cooked. Over time, the cracked and fire-shattered stones were discarded to the sides, gradually building up the distinctive horseshoe shape that survives today.

The mound at Kilnahulla Beg measures 14.4 metres north to south and 12 metres east to west, rising to a height of 1.5 metres, making it a reasonably substantial example. Its opening, approximately 2.7 metres wide, faces west-northwest, the gap in the horseshoe indicating where the trough would once have sat. The proximity to the stream, roughly ninety metres to the south, is entirely typical of the type; fulachtaí fia are almost always found near a reliable water source, and their distribution across the Irish landscape closely follows the pattern of rivers, streams, and boggy ground. Most date to the Bronze Age, broadly between 1500 and 500 BC, though some sites have earlier or later phases of use.