This is a fulacht fia, and the fact that it looks like little more than a grassy hump is entirely typical of the type.

Fulachtaí fia are ancient cooking, or possibly bathing and industrial, sites found in their thousands across Ireland. The core feature is a mound of burnt and fire-cracked stones, accumulated over repeated use. The basic method involved heating stones in a fire and dropping them into a water-filled trough, bringing the water rapidly to the boil. The stones, fractured by repeated heating and cooling, were discarded to the sides, building up the characteristic horseshoe-shaped or oval mound over time. Most examples in Ireland date to the Bronze Age, roughly 1500 to 500 BC, though some have earlier or later phases. The Ballymacphilip example is described simply as a heavily overgrown mound of burnt material, sitting in pasture, which is precisely the condition in which many of these sites survive across Cork and beyond. North Cork has a notable concentration of them, partly a reflection of the landscape and partly of how thoroughly the county has been surveyed.