This National Monument, protected since 1976, presents a fascinating puzzle of Irish defensive architecture that may span from pre-Norman times through to the late medieval period. The castle’s most striking feature is its massive curtain wall, built with cyclopean masonry blocks so large they caught the attention of 19th-century antiquarians. The wall, measuring 2.4 metres thick and originally standing 4.6 metres high, features unusual shouldered window embrasures that suggest pre-Norman construction. Rather than forming a complete enclosure, this wall appears to have functioned as a single defensive barrier cutting off access to Knockadoon, much like a promontory fort, with Lough Gur’s waters providing natural protection on the remaining sides.

Access to the castle was remarkably elaborate, achieved via a stone causeway that once stretched across the marshy waters when the lake level was higher. This impressive structure, approximately 132 metres long and 6.7 metres wide, connected two gatehouses; a now-levelled square castellated gateway to the south and a surviving gatehouse projecting from the curtain wall. The causeway, depicted in Thomas Dineley’s 1680 drawing as ‘The long bridge with Drawbridges’, was the sole approach to Knockadoon. The surviving gatehouse features gun loops providing flanking fire along the curtain wall, whilst inside, evidence of wicker centring on the arch’s underside and wooden beam holes reveal medieval construction techniques. A cobbled roadway, still visible inside the gateway, once led around the base of Knockadoon towards Lough Gur Castle, located 575 metres to the northeast.

The rectangular tower that gave the castle its name was added to the southwest angle of the curtain wall during the medieval period, creating the structure known as the Black Castle. With internal dimensions of approximately 7.6 by 4 metres and walls 2.3 metres thick, the tower featured a vaulted ground floor ceiling. Historical accounts from 1826 describe the defensive wall as being ‘made up of immense blocks of stone, they are the largest that can be seen in any building of this kind in the county’. The castle’s strategic position, combined with its sophisticated causeway system and multi-period construction, suggests Knockadoon held significant importance throughout centuries of Irish history, from possibly serving as a pre-Norman dún to its later medieval incarnation as a formidable lakeside fortress.