It belongs to a broad category of field monument that turns up across Ireland in considerable variety, ranging from the raised rims of ancient enclosures to the eroded banks of ringforts, cultivation ridges, or boundary works whose original purposes have long since blurred. The term earthwork, in an archaeological context, simply means any feature formed by the deliberate shaping of soil or earth, whether as a wall, a ditch, a mound, or a platform. That Castlereagh has one is noted; what precisely it represents remains, at least in any publicly available form, unrecorded.

Castlereagh as a place-name has some resonance in Mayo, and earthworks of various kinds in the west of Ireland span an enormous chronological range, from prehistoric enclosures of the Neolithic and Bronze Age through to post-medieval field systems and the landscape scars left by plantation-era reorganisation. Without specific detail attached to this particular monument, it is not possible to say which tradition it belongs to, or what survives above ground. It is the kind of site that rewards careful attention in the field even when documentary sources are thin, since the shape and orientation of an earthwork, and its relationship to surrounding townland boundaries or water sources, can suggest a great deal about its original function.