The earthwork that once defined this early medieval enclosure has been reduced, over centuries of farming, to a pattern readable only from the air. Aerial photographs reveal a cropmark, the faint signature left when buried archaeological features cause crops above them to grow at slightly different rates, tracing the full circuit of a fosse, a defensive ditch, that once enclosed an area roughly 55 to 60 metres across. It is a considerable size for a rath, the Irish term for a ringfort, the enclosed farmstead type that served as the basic unit of settlement throughout early medieval Ireland.

The site sits towards the lower portion of a west-facing slope, a position that would have offered practical advantages in terms of drainage and aspect. What the cropmark also preserves, along the western to north-western arc, are traces of a narrow outer bank, around three metres wide, accompanied by a second ditch beyond it. This double-circuit arrangement, an inner fosse with an additional external bank and ditch, suggests a site of some consequence. Ringforts with multiple enclosing features are generally associated with higher-status occupants in the early medieval period, when such earthworks were raised by hand and represented a substantial investment of labour. The landscape around Castlehaystown has long since absorbed the fort back into agricultural use, but the geometry of the original enclosure remains locked into the subsoil, surfacing only when the right crop, the right season, and the right angle of light coincide to make it legible again.