It is not the entrance to anything built in living memory, nor in medieval times; the copper mine at Mountgabriel is thought to be prehistoric, part of a cluster of Bronze Age workings that made this corner of Ireland one of the earliest centres of metal extraction in north-western Europe.

The tunnel itself runs at roughly 275 degrees into an east-facing rock scarp, driven with tools that were not made of iron or steel. Stone mauls, the rounded hammerstones used to pound and fracture ore-bearing rock, have been found exposed in section in the water-eroded channel that runs outside the entrance. These mauls are one of the clearest signatures of prehistoric mining; Bronze Age miners had no metal picks, so they used hand-held or hafted stone weights, sometimes combined with fire-setting, to break rock and extract copper ore. The interior of the tunnel is now waterlogged and contains what appears to be a peat deposit, and it has not been excavated. Probing has confirmed a length of at least three metres beyond the entrance, though the full extent remains unknown. Outside, a large semi-circular spoil mound, roughly fifteen metres north to south and thirteen metres east to west, marks where extracted material was piled over generations of work. A water-eroded channel, over seven metres long, runs nearby. The site was catalogued by O'Brien in 1987 as mine 29 in his survey of the Mountgabriel complex.

The entrance dimensions, one metre wide and just thirty centimetres high, are a reminder of how physically demanding this work must have been. Whoever quarried here was working on their belly or their knees, in the dark, with stone. The monument is protected under the National Monuments Acts, which means the site and its remaining deposits, including those unexcavated peat layers that may yet preserve organic material or wooden tools, are legally safeguarded against disturbance.