Most early enclosures in Ireland are roughly circular, the product of a ringfort tradition stretching back through the early medieval period, so the trapezoidal outline here is already a little out of the ordinary. The earthwork measures roughly 26.5 metres east to west and narrows from about 15 metres wide at the western end to just 10 metres at the eastern end, the whole thing defined by a scarp that reaches up to 3.6 metres in height. The entrance sits at the west, where the bank drops to its lowest point, and the interior is now heavily overgrown with trees and undergrowth.

What makes the place more than just a curiosity of geometry is what occupies the enclosed ground: a children's burial ground. These sites, known in Irish tradition as cillíní, were used for the interment of unbaptised infants and others considered ineligible for consecrated ground, including stillborn children and sometimes adults who died outside the Church. They are found across Ireland in their hundreds, often tucked into older earthworks, boundary margins, or liminal landscapes, places that sat at the edge of the parish's spiritual map. The enclosure at Letter was already being recorded by the time of the 1846 Ordnance Survey six-inch map, which marks it as an embanked trapezoidal enclosure, so the physical structure is demonstrably old, though the origin and original purpose of such an unusual form remain unclear. The ridge-top position, overlooking the broad valley below, would have made it a conspicuous feature in the landscape for a considerable period before that survey was made.