The deviation is modest, barely noticeable at speed, but it exists because the road was laid out around the northern edge of something older and, in the minds of those who planned the route, apparently worth avoiding. That something is a rath, a ringfort, sitting on the crest of a broad, flat ridge above gently undulating pastureland at Arbourhill.

Ringforts are enclosed farmsteads of the early medieval period, typically dating from somewhere between the fifth and twelfth centuries, and they survive in the Irish countryside in extraordinary numbers, several tens of thousands in various states of preservation. This one at Arbourhill is roughly circular, measuring just over thirty-two metres north to south and thirty-one metres east to west. It is defined by a broad, flattened bank, between thirteen and nearly eighteen metres wide in places, and an outer fosse, which is simply the ditch dug to provide the material for the bank. The interior height of the bank is quite slight, under thirty centimetres in places, though the exterior face still rises to around seventy-five centimetres above the base of the fosse. A gap of about four and a half metres in the south-south-east quadrant marks what was likely the original entrance, and there is a faint suggestion of an outer bank surviving in the south quadrant. The whole structure sits under pasture now, low and weathered, the kind of earthwork that rewards a second look rather than announcing itself.

One small detail gives the site an oddly domestic quality alongside its antiquity: a decommissioned ESB pole stands on the bank in the north-west quadrant, a remnant of twentieth-century rural electrification that has simply been left in place. It is a peculiar juxtaposition, early medieval enclosure and mid-century infrastructure sharing the same earthen ridge, both now equally out of service.