This is a fulacht fia, a type of ancient cooking site found in considerable numbers across Ireland, typically identified by a crescent or horseshoe-shaped mound of fire-cracked stones beside a water source. The mound at Tombay measures roughly 16 metres north to south and 13 metres east to west, rising between half a metre and a metre above the surrounding ground, and opens to the west.

Fulachtaí fia, as they are known in the plural, are generally dated to the Bronze Age, though some continued in use into later periods. The working principle was straightforward: stones were heated in a fire, then dropped into a water-filled trough to bring it to the boil, and the shattered, heat-spent stones were discarded to the sides, gradually building up the characteristic mound over repeated use. The proximity to water was essential, and at Tombay a stream running roughly west-northwest to east-southeast lies about 70 metres to the north, close enough to supply the site without difficulty. The mound does not appear on earlier Ordnance Survey mapping and was first recorded on the 1940 edition of the OS six-inch map. The earliest published reference is by Ranson in 1945, which suggests the site attracted at least some local or antiquarian attention in the mid-twentieth century, even if it has since slipped from wider view. The valley-floor location, with its low-lying, potentially marshy ground, is entirely typical of where these sites tend to cluster.