The rath at Treanfohanaun in County Mayo has been levelled to the point where only a faint arc of compressed ground betrays its original shape, yet that shape was once clear enough to be mapped in detail. A rath, sometimes called a ringfort, is a roughly circular enclosure defined by one or more earthen banks and ditches, typically built in early medieval Ireland as a farmstead and symbol of status for a family of some means.

The Ordnance Survey's six-inch map of 1838 recorded it as a complete circular embanked enclosure, suggesting it was still reasonably legible at that time. By the 1931 edition, the depiction had changed: the cartographers now showed a penannular form, meaning a ring broken at one point, open to the north, with a diameter somewhere between twenty-five and thirty metres. What survives today is a semi-circular arc of a levelled scarp, roughly twenty-four metres across, traceable from the south around to the northwest. The most substantial remnant sits at the northwest, where the ground still rises to about 0.6 metres and the old perimeter merges into a field fence of earth and stone that curves gently northward. The rest has been absorbed into the surrounding pasture, which occupies a low rise with a gradual fall to the west. From that western aspect, the land opens towards the confluence of the Pollagh and Glore Rivers with the Gweestion River, a landscape detail that may well have informed the original choice of site, since such positions offered both visibility and an awareness of movement through the surrounding countryside.