It is a fulacht fia, a type of prehistoric cooking site found in great numbers across Ireland, and its presence on this small Shannon estuary island is a reminder that these apparently remote places were once part of a settled, active landscape. The mound measures roughly 8.5 metres by 9.6 metres and stands just 0.7 metres high, modest enough to be mistaken for a natural rise in the ground.

Fulachtaí fia, the plural form, are among the most common archaeological monuments in Ireland, with many thousands recorded. The basic principle involves heating stones in a fire, dropping them into a water-filled trough to bring it rapidly to a boil, and using the resulting heat for cooking, and possibly for bathing or industrial processes such as working leather. Over repeated use, the stones shatter and become unusable, and are discarded to form the characteristic horseshoe or kidney-shaped mound of scorched debris that survives today. The Foynes Island example follows this familiar form, with its opening, about 2.5 metres wide, facing west-north-west, likely indicating where the trough once sat. The record was compiled by Denis Power and uploaded in August 2011.

Foynes Island sits in the Shannon estuary and is not straightforwardly accessible, so visiting requires advance planning around access to the island itself. The monument lies within a coniferous plantation, which means light levels are low and ground vegetation is often sparse, conditions that can actually make low earthworks easier to read once your eyes adjust. The mound's dark, stony composition distinguishes it from the surrounding soil if you get close enough. There is no dramatic feature to photograph, but the scale of the thing, barely knee-height, spread across nearly nine metres, conveys something about the accumulation of repeated, ordinary effort over a long stretch of prehistoric time.