A rath is an earthen ringfort, typically a circular bank and ditch enclosing a farmstead from the early medieval period, and thousands of them survive across Ireland. This one, however, was unusual from the start: it sat in marshy ground, a setting that would have made it awkward to defend or farm, and it left behind no clues about who built it or when.

In 1939, ahead of the flooding of the Poulaphouca Reservoir, which submerged a considerable stretch of the King's River valley and displaced several townlands, archaeologists excavated the site at Lackan. The circular enclosure measured roughly forty metres in diameter. Despite the excavation, no internal features were found and nothing dateable came to light. The work was later reported by O'Connor in 1944, but the site itself had already been consigned to the water. It now exists only as a record: a shape in the ground that was examined, found silent, and then lost.

There is nothing to see at Lackan today, which is itself a kind of point. The reservoir, completed in the early 1940s, covers a landscape that once held farms, roads, and at least one ancient enclosure that resisted interpretation even when archaeologists had the chance to ask it questions directly.