The enclosure at Willowbrook in County Sligo belongs firmly to the second category. By the time anyone thought to record it in any detail, it had already been erased, and what we know of its shape comes not from anything you could touch or photograph but from a hachured arc, a short arc of shading used by cartographers to suggest a raised or earthen feature, appearing on the 1913 edition of the Ordnance Survey six-inch map. That arc measured roughly 25 metres in length. The 1837 edition of the same map shows nothing at all.

The gap between those two surveys is telling, though not in the way one might expect. The monument does not appear between 1837 and 1913 because it was built in that period; enclosures of this kind in Ireland are typically early medieval or prehistoric in origin, defined by a bank or ditch that once enclosed a dwelling, a farmstead, or a space with ritual significance. What changed between the two map editions was simply the cartographers' attention, or perhaps the condition of the earthwork at each moment of survey. The site itself sits on a slight terrace in gently undulating pasture, with a steep south-facing slope dropping away to the south, a position that would have made practical sense for a settlement, offering some shelter and a degree of natural defence or drainage. That topographic logic is now all that remains to suggest why someone once chose this particular patch of Sligo ground.