Growing in an arc across flat Limerick pasture, they trace the ghost of a fosse, the shallow external ditch that once ringed an early medieval farmstead. The ditch itself is barely there, just ten centimetres deep and roughly two and a half metres wide, but the thistles seem to have found something in that slight depression worth returning to, season after season, and in doing so they have preserved the outline more legibly than the earthworks alone could manage.

The site is a rath, the most common type of ringfort in Ireland, typically a circular enclosure of earthen banks within which a farming family would have lived during the early medieval period, roughly 500 to 1000 AD. This particular example in Rooskagh East was recorded as an embanked circular enclosure on the Ordnance Survey six-inch map of 1841, meaning it was still a recognisable monument at that point. Since then, most of it has been levelled, though a section of the enclosing bank survives along the north-west to east arc, standing 0.55 metres on its interior face and 0.9 metres on the exterior. A modern east-west field boundary now crosses the northern part of the enclosure, cutting through what would once have been the interior of the settlement. The record was compiled by Denis Power and uploaded in August 2011.

The interior, measuring approximately 25.5 metres from the field boundary to the southern bank and 28 metres east to west, is boggy underfoot and dips gently towards the centre, so firm footwear is advisable. The setting is entirely unassuming, low-lying pasture with no particular drama to the landscape, which makes the thistle arc all the more worth looking for. It runs just outside the surviving bank section, and in late summer when the plants are in flower it is at its most conspicuous. There is no formal access or signage, and the land is agricultural, so any visit should be made with appropriate consideration for the working farm around it.