The earthwork in Thomastown townland, in the barony of Coshma, measures roughly 40 metres on its northeast to southwest axis and about 30 metres across from northwest to southeast, defined by a scarp edge and a fosse, the term for the shallow ditched enclosure that typically borders earthen structures of this kind. It is modest in scale, unassuming in appearance, and easy to miss unless you know to look for the slight rise and the dip at its margins.

What makes this site particularly curious is how thoroughly it escaped the attention of early cartographers. When the Ordnance Survey of Ireland produced its first detailed six-inch maps in 1840, the earthwork was not recorded at all. It only appears on the later 25-inch edition of 1897, where it is shown as a raised oval area with the surrounding fosse running from the southeast, around the west, and back to the northeast. Whether it was genuinely overlooked during the earlier survey or had not yet been sufficiently exposed by then is not clear from the available record. A possible ringfort lies roughly 250 metres to the west, catalogued under the Sites and Monuments Record reference LI047-041, and the two features together suggest this small corner of the Coshma barony may have seen more organised, settled activity at some point in the past than the present landscape implies. The earthwork sits about 100 metres east of the boundary with the adjoining townland of Mountblakeney.

For anyone wanting to locate the site today, modern satellite imagery offers the clearest view. The outline of the fosse remains visible on Google Earth orthoimages, though a farm track running northwest to southeast cuts across the eastern side of the earthwork, a small intrusion that also helps fix its position in the field. As with many such features in reclaimed agricultural land, the ground-level experience is subtle; the raised area and the fosse depression are easier to read from above than on foot. Visiting in late winter or early spring, when grass growth is lower, gives the best chance of making out the landform in person.