Nobody knows precisely when it was built or by whom. The date is recorded simply as indeterminate, which places it in a particular category of Irish archaeological remains: present enough to be surveyed and mapped, absent enough to resist interpretation.

What survives amounts to a sunken rectangular area, roughly ten metres along its longer north-north-west to south-south-east axis and four metres across, defined on three sides by low earthen banks covered in sod. The banks on the south and west sides are the clearest, standing between half a metre and just under three-quarters of a metre above the interior floor level, though they barely protrude above the surrounding ground on the outside. The eastern side retains only degraded remnants, but there is a gap of about four metres near the southern end that may once have served as an entrance. At the northern end, a slight rise in the ground and a change in vegetation hint at where the enclosing bank continued, though the evidence is too faint to measure with confidence. The western bank has been further eaten away by coastal erosion at its northern end, and the process is presumably ongoing. A field boundary sits just over three and a half metres to the south-east, suggesting this structure existed within some broader pattern of land use on the island, even if that pattern is now equally difficult to read.

Bartragh Island sits in Killala Bay off the coast of County Mayo, an area associated with the French expeditionary landing of 1798, though nothing in what remains of this particular structure connects it to that or any other documented event. It simply occupies its exposed position at the end of a peninsula, grass-covered and level inside, eroding quietly at the edges.