It is, in fact, the remnant of a fulacht fia, one of the most widespread and still somewhat mysterious monument types in the Irish countryside.

A fulacht fia is a burnt mound, the accumulated debris of a prehistoric cooking or industrial process. The typical method involved heating stones in a fire and dropping them into a water-filled trough until the water boiled, a technique that leaves behind a characteristic crescent or kidney-shaped heap of shattered, heat-stressed stone. The Loumanagh example measures roughly thirteen metres north to south and fifteen metres east to west, rising to a modest height of 0.64 metres, with an opening about four metres wide facing west-northwest, the gap marking where the working area and trough would once have been. Such monuments date broadly to the Bronze Age, though they appear across a wide span of prehistory, and thousands have been recorded across Ireland, concentrated along river margins and boggy ground where water was reliably accessible. Their precise function has been debated at length; cooking remains the most widely accepted explanation, though possibilities ranging from textile processing to communal bathing have also been proposed.

The mound at Loumanagh is unexcavated and sits in ordinary farmland, which means there is little to indicate its age or original context beyond its shape and composition. That kidney form, with the characteristic hollow opening, is itself the diagnostic detail worth understanding before you encounter one of these sites. Seen without that context, it is easy to walk past entirely.