Built around 1220 by the Anglo-Norman de Lacys during their ambitious attempt to conquer Bréifne, the castle sits on what was once the royal crannog of the O'Reilly clan, who swiftly reclaimed it in 1233. For nearly four centuries afterwards, this formidable fortress served the O'Reillys more as a military stronghold and prison than a comfortable residence, its 2.5-metre-thick walls rising over 18 metres high from the water's edge.

The castle's most dramatic period came during the turbulent 17th century. Following its capture by government forces in 1608, it underwent extensive renovation as part of the Plantation of Ulster, complete with a new portcullis-protected entrance, five storeys of wooden floors, and numerous windows; transforming it from a medieval keep into a more habitable, if still imposing, structure. During the 1641 rebellion, it became a prison for notable figures including William Bedell, the Protestant Bishop of Kilmore, whilst Owen Roe O'Neill, the victorious Confederate commander at the Battle of Benburb, died here in 1649. The castle's military significance ended spectacularly in 1653 when, after holding out as the last fortress against Cromwell's forces in Ireland, it finally surrendered following a six-week artillery siege that left cannon balls and mortar bombs scattered throughout its interior.

Archaeological excavations in 1987 revealed a treasure trove of 17th-century artefacts beneath the rubble: clay pipes, harp tuning pegs, book mounts, and the hastily buried remains of four siege casualties. The finds paint a vivid picture of life during the castle's final active years, from everyday items like cooking pots and candlesticks to the grim reminders of warfare in broken weaponry and ammunition. Now a protected National Monument in state care, the castle's ruined walls continue to rise defiantly from their island perch, bearing witness to centuries of conquest, rebellion, and the explosive finale that left this once-mighty fortress permanently scarred.