This one in Fosterstown South, on the northern fringes of County Dublin, exists primarily as a ghostly outline in a field, visible only from the air, where a difference in crop growth betrays the buried ditches of an enclosure that has otherwise vanished from the surface entirely. It is the kind of site that reminds you how much of early Ireland is still legible, just not at eye level.

What the aerial photographs revealed was not a simple circular enclosure but a bivallate one, meaning it was ringed by two concentric ditches rather than one. A ringfort, to give the broader context, was typically a circular farmstead of the early medieval period, enclosed by one or more earthen banks and their accompanying ditches, used to protect a family and their livestock. The bivallate variety, with its doubled defences, is generally considered a marker of higher social standing. At Fosterstown South, the inner enclosure measures roughly 30 metres in diameter, partially wrapped on its eastern, northeastern and northern sides by a second, elliptical outer ditch some 50 metres across. Crop marks also suggest an associated field system lying to the south, and a separate enclosure has been identified further in that direction. When Metro North infrastructure planning prompted investigative work in the area, geophysical survey and test excavation confirmed what the aerial evidence had implied. No diagnostic finds were recovered during the excavation, so the site cannot be dated precisely, but the overall form is consistent with a bivallate ringfort, as noted by Hession in 2009.

Because the enclosure survives below ground level rather than as an upstanding earthwork, there is nothing obvious to see from the roadside. The crop mark that first identified the site is the kind of thing best appreciated through the aerial photography held in the Historic Environment Viewer, where the outlines become clear. Anyone with a particular interest in early medieval settlement patterns or in how Metro North-related development archaeology brought dozens of such sites to light across north County Dublin will find the records worth exploring through the Sites and Monuments Record entry DU011-117----. The field itself is on private agricultural land, and the most productive way to engage with this site is through those archival and digital resources rather than a visit in person.