To a casual observer it looks like little more than a shallow dip in a field, averaging perhaps two metres wide and barely forty centimetres deep, but its geometry is deliberate and its origins are early medieval. This was a souterrain, an artificially constructed underground chamber or tunnel, typically built from drystone walling and roofed with large capstones, which served the inhabitants of a ringfort as a place of refuge, food storage, or concealment. The collapse or removal of those structural elements over centuries is what leaves the distinctive hollow visible today.

The plan that survives measures 8.5 metres along its main east-west axis, with a second arm of 4.5 metres branching off northward towards the eastern end, giving the whole thing its T-shaped form. It sits within a ringfort, the circular enclosed farmstead that was the standard unit of rural settlement in early medieval Ireland, typically dating from roughly the sixth to the twelfth centuries. Souterrains were a common feature of such sites across the country, though they vary considerably in scale and complexity. The example at Carrowrevagh More is modest by those standards, but the clarity of its layout, even in its ruined state, makes the original intention readable. The branch passage in particular is a detail worth noting; the forking of a souterrain into separate arms is a design seen elsewhere and may have served to complicate the passage for anyone who did not already know its layout.