The structure is modest to look at: a rectangular run of contiguous slabs about four metres long and less than a metre wide, with one end splayed outward in a slightly bulbous widening. The capstones are gone, at least one slab has toppled, and the whole thing is yielding, slowly, to erosion. Yet the form is specific enough to suggest a purpose: the remains are interpreted as a hearth and flue, the kind of arrangement used in early metalworking, and the tidal encroachment is gradually erasing whatever evidence remains at the north-east end.

What gives the site its real interest is a parallel found roughly 650 metres to the south. During excavation of a hut site at Glannafeen, a comparable structure was uncovered and interpreted as an iron smelting site, a process in which iron ore is heated with charcoal to extract usable metal, typically within a simple stone-built furnace or hearth with a directed flue to concentrate the heat. That excavation was published by Ó Cuileanáin in 1955, and the resemblance between the two structures suggests this corner of Lough Hyne may once have supported small-scale metalworking activity. Two other features lie nearby on Castle Island itself: a cupmarked stone, one of those flat or outcropping rocks bearing shallow circular depressions whose age and meaning remain debated, and a possible earthwork, both within about twenty metres to the east and south.