It is only visible at all because the land has been cut away into an almost vertical scarp, exposing the midden in cross-section like a slice through time. A midden is, in its simplest form, a refuse heap, the accumulated remains of meals eaten by people who lived, worked, or gathered nearby. What looks like an unremarkable bluff at the water's edge turns out to be a compressed archive of shellfish consumption.

The deposit runs roughly five metres along a north-northwest to south-southeast axis and reaches about thirty centimetres in depth. Oyster shells make up the bulk of it, densely packed and sitting in a matrix of grey-brown soil, with cockle shells and the occasional periwinkle mixed in. The site lies in the estuary of the River Moy, a stretch of water long associated with fishing and with the movement of people and goods along the Mayo coast. Middens like this one appear all around the Irish coastline and along tidal rivers, left by communities for whom shellfish represented a reliable, accessible food source. The precise date of this particular deposit is not recorded, but shell middens in Ireland range from prehistoric to early modern periods, and without excavation it is difficult to place this one more narrowly than that.

Bartragh Island itself sits in the lower Moy estuary and is accessible only by water or at low tide across tidal flats, which means the midden exists in a landscape that already demands some effort to reach. The peninsula where it sits is narrow, and the exposed scarp where the shells are visible faces west, into the estuary. For anyone who does make it out there, the detail to look for is at the top of that cut bank, where the dark, shell-dense layer sits directly beneath the thin strip of living sod above it.