The one at Carrowkeel in County Mayo presents a quiet puzzle: it is D-shaped rather than round, or at least it appears that way now, and no one is entirely certain whether that was always the intention. Sitting on a gentle rise at the edge of low-lying wet ground, it was absent from the Ordnance Survey's six-inch map of 1838, only appearing, as a partial hachured arc, on the 1917 edition. Whether it was simply overlooked by the earlier surveyors or had not yet been sufficiently exposed by then is an open question.

The enclosure measures roughly 36 metres east to west and 26.6 metres north to south. Its curved southern side is defined by a combination of scarps and an earthen bank, outside which runs a flat-bottomed fosse, a defensive ditch approximately 2.8 metres wide, backed by an outer bank about 3.6 metres across. The northern side, which would complete the circle if the enclosure were originally round, is marked by nothing more than a collapsed east-west field wall reduced to a low scatter of large stones and boulders. The interior slopes downward toward that northern edge and continues falling gently toward the wet ground beyond, with no trace of any enclosing element surviving on that side. A break of around four metres in the bank to the east-southeast may represent the original entrance, with a faint suggestion of a causeway crossing the fosse at that point. In the western interior, close to the bank, a shallow depression roughly five or six metres across lies partly hidden beneath a tangle of brambles and blackthorn. The outer bank itself has been absorbed at both ends into modern field fences, and is densely overgrown with hazel, hawthorn, and blackthorn, giving the whole southern arc a distinctly unkempt, slowly vanishing quality. Whether the D-shape is original design or simply the southern half of something larger that was lost to time and waterlogged ground remains genuinely unresolved.