But the souterrain at Fortwilliam in North Kerry is precisely the kind of structure that rewards a second look, even from the outside. A souterrain is an underground passage or chamber, typically built during the early medieval period in Ireland, usually associated with a nearby settlement and thought to have served as a place of refuge, storage, or concealment. At Fortwilliam, the entrance is framed by five large stones, and the passage itself is built of drystone walling, that is, stones laid without mortar, relying on careful placement and weight distribution to hold their form. What direction it runs can only be estimated; the best assessment is that it heads roughly northward, though the collapse of the tunnel makes it impossible to confirm.

The site was recorded as part of the North Kerry Archaeological Survey, published in 1995 by C. Toal, which catalogued a wide range of monuments across the region. The fact that it appears nowhere on any edition of the OS maps suggests it was either overlooked during successive surveys or simply not considered significant enough to mark. The cross-reference to a site at Kill nearby hints at a wider pattern of early activity in the area, though the details of that connection remain unresolved. Structures like this one, modest in scale and easy to miss, are often the last traces of communities that left little else behind.