In a field near Ballyvass in County Kildare, a circle buried beneath the soil betrays itself only from the air. An aerial photograph captures what is known as a cropmark, the faint but legible impression left when buried features such as ditches cause the vegetation above them to grow differently, usually greener and taller over filled-in ditches, or paler and shorter over buried walls or compacted ground. Here, the cropmark traces the line of a fosse, an encircling ditch, forming a roughly circular enclosure with an estimated maximum diameter of around 25 metres.

What lies beneath that ditch is not yet certain. The enclosure could be a ringfort, the most common monument type in the Irish countryside, typically a farmstead of the early medieval period enclosed by one or more earthen banks and ditches. Alternatively, it may be a ring-barrow, a funerary monument of broadly similar circular form but associated with burial rather than habitation, and generally of prehistoric date. The two can look remarkably alike at ground level, and even from the air the distinction is not always easy to draw. At 25 metres in diameter, the Ballyvass enclosure sits within a size range that could accommodate either interpretation, and without excavation or further survey the question remains open.