On an east-facing slope of undulating pasture at Castletown in County Sligo, a field system was catalogued as a potential monument, yet when the ground was inspected in 2003, there was nothing to see. No earthworks, no boundary traces, no humps or hollows in the grass.

A field system, in the archaeological sense, refers to the fossilised remains of ancient land division, the banks, ditches, and lynchets left behind when people organised and worked a landscape over generations. Such features can survive for centuries as faint surface traces, only to be lost to agricultural improvement, drainage works, or simple erosion. The Castletown site has a quietly complicated administrative history of its own. It was absent from the Sites and Monuments Record compiled in 1989, then included in the revised Record of Monuments and Places in 1995, yet it never appeared on any edition of the Ordnance Survey six-inch map. That sequence, recognised but unmapped, present in one record and absent from another, suggests the site existed primarily as a suspicion, something glimpsed perhaps in aerial photography or in the memory of an older landscape, but never firmly anchored to the ground.