Promontory fort - coastal, Shenick'S Island, Co. Dublin

A straight fosse, roughly two metres wide and flanked by a low earthen bank, cuts across the island at its narrowest point, dividing the northwestern third from the rest of the landmass. The feature looks, at first glance, like the defining characteristic of a promontory fort, a type of prehistoric enclosure in which a headland or spit of land is sealed off by a ditch and bank, using the sea itself as a natural defence on the remaining sides. But the island's topography complicates that reading, and no one has yet dug a spade into the ground to find out for certain.

The site was described by Casey in 2003, who noted that while the fosse and bank are clearly present, the place cannot be confidently identified as a defended promontory. Within the area cordoned off by the earthwork, the only features that remain legible above ground are a Martello tower, one of the squat circular gun towers built along the Irish coast in the early nineteenth century as a defence against potential Napoleonic invasion, and several earthworks that may represent levelled field boundaries, though even these are uncertain. The ambiguity is not unusual for sites of this kind; coastal erosion, centuries of grazing, and later land use can reduce early medieval or prehistoric features to near-invisibility, leaving only the broadest outlines behind.

Shenick's Island is a tidal island, accessible on foot at low tide across a causeway from the Skerries shoreline, though the crossing requires attention to tidal times and can be slippery underfoot. The Martello tower is the most immediately visible structure once you reach the island, and the fosse and bank are easier to trace on the ground than they might appear on a map, crossing the narrower neck of land to the northwest. There has been no formal archaeological excavation of the site to date, which means the question of whether the earthwork is genuinely prehistoric, early medieval, or something else entirely remains open. For anyone with an interest in the gaps in the archaeological record, that uncertainty is itself a reason to look carefully.