At low tide, when the water retreats to the north-east, a strip of access opens up and the island becomes briefly reachable on foot. It is on this eastern, landward side of the island that an early medieval cashel quietly occupies a south-east-facing slope of good meadowland, overlooking nothing more dramatic than grass and tidal mud. The combination of island setting and ancient enclosure is unusual enough; what makes it stranger still is how thoroughly time has softened the evidence.

A cashel is a type of ringfort built primarily from stone rather than earth and timber, typically constructed during the early medieval period in Ireland as a defended farmstead or high-status enclosure. The one on Scanlan's Island is oval in plan, measuring roughly 50 metres north-west to south-east and just over 34 metres across its shorter axis, though these figures trace the spread of collapsed material rather than any standing wall. The structure has been reduced to a low, grass-covered stony bank, with outer facing-stones still just visible on the west-south-west side, the last legible fragments of what was once a more substantial perimeter. Inside the south-western quadrant sits a rectangular building of indeterminate date, its origins unresolved. A modern field wall cuts across the site from north-west to south-east, absent from the 1915 Ordnance Survey six-inch map, which means it was added sometime in the twentieth century, bisecting the ancient enclosure without apparent concern for what lay beneath it.

Access to the island depends entirely on the tides, and the cashel is best approached from the north-east when the water is out. Once there, the earthwork is easy to miss at first; what reads on a plan as a substantial oval enclosure presents on the ground as a gentle grassed-over rise, its geometry only becoming legible when you move around its circumference and notice the low scarp at the south-east and the scatter of exposed stones at the west.