The rath at Cartron sits on a rise in County Mayo, and was originally positioned to overlook a bend in the river roughly seventy metres to its south. That bend no longer exists in quite the same place; the river's course has since been diverted, and the water now runs further south than it once did. The fort still commands its elevated position, but the view it was built to survey has shifted beneath it.

A rath is an earthen ringfort, the most common type of early medieval enclosed settlement in Ireland, typically consisting of a raised circular or oval area surrounded by a bank and ditch. This one is broadly oval, measuring approximately 25.5 metres on its northwest to southeast axis and 20.5 metres across. The enclosing element here is an earthen scarp rather than a freestanding bank; on the southwest side it rises to an external height of around 2.4 metres, while the northeast is somewhat lower at 1.4 metres. The southwest is also notably wider in slope, at 4.6 metres, compared to 2.5 metres at the northeast. This asymmetry is partly deliberate and partly topographical: on the south to northwest arc, the builders made use of the natural fall of the ground rather than constructing the full height artificially. A low bank runs along the top of the scarp on that same arc, though this may belong to a later field boundary rather than the original structure. Just outside the scarp on the northeast to southeast side, a slight depression hints at what may once have been a fosse, the external ditch that typically accompanied such enclosures.

The interior is level across the west and centre, dropping gently toward the northeast and more steeply toward the southeast. Thick grass hummocks cover the ground inside, while gorse and broom have colonised the western slope of the scarp. A trench roughly four metres long, two metres wide, and a metre deep cuts through the scarp at the northwest, though its purpose is unclear and vegetation has largely obscured it. Where the original entrance once stood is also uncertain; there are eroded gaps at the north and northeast, and a poorly defined low area at the southeast, but none of these can be confirmed as the intended threshold. A road runs past the fort along its southwest to west-northwest edge, and a field boundary that the 1919 Ordnance Survey map showed intersecting with the southern edge of the rath has since been removed.