But those boulders are the detail that matters. They are thought to be the surviving remnants of an island cashel, a type of stone-walled ringfort built not on solid ground but on an island, natural or otherwise. And in this case, the island itself may not be entirely natural either.

The Ordnance Survey Letters, a series of detailed topographical and antiquarian notes compiled in the nineteenth century as part of the wider mapping of Ireland, record a local tradition that Castle Island was "partly artificial". That phrase, noted by O'Flanagan in the early twentieth century drawing on those earlier surveys, places the site in a category of constructed or substantially modified lake islands. A fully artificial island of this kind would make it a crannog, a platform dwelling common in early medieval Ireland, typically built from timber, stone, and compacted material. Whether this island is a crannog, a cashel built on a natural islet, or some combination of the two has never been firmly resolved. Antiquarians James Hardiman and G.H. Kinahan both noted the site in the nineteenth century, suggesting it was recognised as archaeologically significant well before modern survey methods existed. What stands on the island today is Ballynahinch Castle, a later structure that has complicated any reading of what lies beneath.

The lake setting means the archaeological remains are not easily accessible on foot, and the boulder perimeter is best understood in the context of what it once enclosed rather than what is visible now. The castle above it draws most attention, but the older, quieter question of whether the ground beneath it was built by human hands remains genuinely open.