A souterrain is an underground stone-built passage, typically associated with early medieval settlement in Ireland, used variously for storage, refuge, or both. The one uncovered here was no modest affair: a straight passage running roughly 16 metres in length, nearly two and a half metres wide and close to two metres high, with two side spurs opening into beehive-shaped chambers. It came to light during the construction of the railway line into Navan, when navvies cut through the gently sloping ground on the eastern bank of the Boyne near Athlumney. Despite being exposed and described, it was never excavated in any systematic sense, and its precise position has since been lost.

The account we have comes from Sir William Wilde, writing in 1849 in his study of the Boyne and its tributary the Blackwater. Wilde, better known today as the father of Oscar Wilde and a prominent Dublin surgeon, was also a serious antiquarian, and his record of the Navan find remains the primary source. Alongside the souterrain, the railway works turned up a substantial collection of objects connected with horse harness, including bridle-bits, buckles, head-pieces, and rings in bronze, iron, and silver, as well as human and animal bones. These were donated to the Museum of the Royal Irish Academy and are now held in the National Museum of Ireland. Wilde believed that much of the souterrain's structure had survived the railway cutting intact, which would make it a significant monument if it could ever be precisely relocated. So far, it has not been.