A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period in Ireland and used variously for storage, refuge, or concealment. They are found across the country in their hundreds, usually associated with ringforts or early settlement sites. This one, if it survives at all, sits quietly beneath the foundations of whatever was built above it.

The sole record comes from the scholar R.A.S. Macalister, who noted in 1899 that he had been told by local people of a cave discovered within a group of modern buildings at Fán. He recorded nothing further, and neither has anyone else since. The structure was not excavated, not measured, and not described in any detail. It passed into the archaeological record as little more than a rumour committed to print, a second-hand account of something glimpsed and then built over. Whether the original discovery was a natural fissure or a man-made passage, whether it remains intact or has long since collapsed under the weight of construction, is simply not known.