That small, repurposed stone does a lot of quiet work: it lends weight to a long-standing local tradition that the fort, known in Irish as Caisleán Aircín, was raised using material robbed from the old monastery that stood nearby, a practice common enough in the seventeenth century but rarely as legible as it is here.

The castle was built around 1652 to 1655 by Cromwellian forces, on the seashore at Cill Éinne on Inis Mór, and on the footprint of an earlier castle that already occupied the site. A manuscript plan dating to 1796 to 1798, now held in the Dublin Oireachtas Library, recorded the fort as roughly rectangular in plan, with two square towers facing the sea and two irregularly placed circular towers to landward. By the time any modern visitor arrives, three of those four towers have gone. What survives is a substantial stretch of the north wall, running approximately 63 metres along the rocky foreshore and standing to about 5 metres in height. At the southern end of that wall a small tower remains intact; further north, a water-gate is still visible, surmounted by a machicolation, the projecting parapet feature that allowed defenders to drop stones or other materials onto anyone attempting to force the gate below.

The wall sits directly on the foreshore rocks, which means the tidal zone and the medieval stonework occupy more or less the same space. The water-gate in particular is worth examining closely, as is the gun loop where the cross-slab lintel can still be seen, a detail easy to miss but one that quietly connects this Cromwellian garrison structure to the much older monastic landscape it consumed.