Cliff-edge fort, Cromlin, Co. Leitrim

The natural drop does the work that a constructed wall might otherwise have done, while an earthen bank and a partly rock-cut fosse, a defensive ditch dug partly into the bedrock, complete the circuit on the other three sides. The fosse is most substantial at the western and eastern ends and fades almost to nothing at the north, where a five-metre-wide entrance breaks the line. The interior measures roughly 33.5 metres east to west and 21.5 metres north to south, all of it grass-covered now. It is the kind of place whose engineering becomes clearer the longer you stand in it.

The place-name Cromlin, and the proximity of a holy well dedicated to St Patrick just fifteen metres to the south-east, drew the antiquarian John O'Donovan to a striking conclusion when he was working in the area in the 1830s. He identified this spot with the site of Crom Cruach, a pre-Christian idol described in early Irish sources as an object of sacrificial worship. According to one tradition, St Patrick destroyed the image and then performed baptisms at the nearby well. The source for this story is the Tripartite Life, a medieval biography of Patrick that scholars treat with considerable caution as a historical document. The identification itself has not gone unchallenged. Writing in the early twentieth century, Dalton placed the most likely location of Crom Cruach at a hilltop enclosure in County Cavan, and that attribution is generally regarded as the more plausible one. Cromlin, then, holds a claim that was argued over by antiquarians and has never been fully settled, which is perhaps its own kind of distinction.