A cashel is a type of ringfort defined by a stone enclosure wall rather than an earthen bank, and the one at An Carn Mór Thiar was once a substantial example, roughly 52.5 metres in diameter. That is a considerable footprint in any field, and yet today most of it has been absorbed back into the landscape through agricultural clearance.

When archaeologists first recorded the site in 1983, they found a poorly preserved but still legible monument. A drystone wall survived from the northern arc around through the east to the southwest, while elsewhere the perimeter had subsided into a low, grass-covered stony bank. There was a clear entrance gap on the eastern side, about 2.7 metres wide, and a subrectangular area of collapsed stonework in the southeast quadrant may have corresponded to internal enclosures described by a researcher named McCaffrey in 1952, who noted what appeared to be two such subdivisions, one in each half of the interior. Additional earthworks lay in the field immediately to the southeast. A modern field boundary and a trackway were already cutting through the monument from north to south at the time of that visit. Since then, field clearance has removed most of what remained. Only a single section of the original cashel wall, running between the east-northeast and east-southeast for about fifteen metres, still survives above ground in any recognisable form.