It runs, according to tradition, from the southern half of a low oval earthwork, heading south-east into the adjoining field. Whether the tunnel was ever properly explored or documented is unclear, but the oral account points to a souterrain, an underground stone-lined passage typically associated with early medieval ringforts and used for storage, refuge, or both. The earthwork itself is a rath, a type of circular or oval enclosure defined by a bank and ditch, built in Ireland broadly during the first millennium AD as a farmstead or settlement boundary.

The rath at Elmhill is modest and worn. It measures roughly 27 metres east to west and 18 metres north to south, making it a relatively small example of the form. What defined it originally was a low scarp and an external fosse, the ditch outside the bank, though this fosse now survives only along the southern arc. A field wall has been built across the northern side, cutting through the enclosing element entirely. Claffey, writing in 1983, recorded the site in this condition, already poorly preserved, its outline readable more as a slight rise and hollow in the grassland than as anything architecturally legible. The survival of the local tradition about the souterrain is, in that context, its most vivid feature, a piece of local knowledge filling in what the ground itself no longer clearly shows.