They dug a fosse, that is a defensive ditch, threw up an earthen bank, and then, because the slope ran the wrong way, quietly raised the interior ground level so that the enclosed space would sit relatively flat. The result is a rath, the Irish term for an earthen ringfort, a type of enclosed farmstead that was common across Ireland during the early medieval period, roughly 500 to 1000 AD. Most were the homesteads of farming families rather than military fortifications, the bank and ditch serving as much to keep livestock in as enemies out.

The Teernaboul example is a well-proportioned specimen. It measures roughly 43 metres across on its northeast to southwest axis and 42 metres on the perpendicular, making it close to circular. The earthen bank survives to an internal height of around 0.8 metres and an external height of 1.8 metres, with a width of 9 metres, giving a sense of just how much material was originally shifted into place. The fosse is more pronounced along the southern arc, where it happens to follow the townland boundary, and shallower to the north. A low rise along the northern arc may represent the remains of an outer bank, which would have added an additional line of definition around the enclosure. The site sits on a southwest-facing slope in pasture, with Mangerton Mountain visible to the south, the kind of long view that would have made the location appealing for settlement in any era.