The castle, which earned its dark nickname from the black limestone used in its construction, sits alongside marshy land that was once part of the lake itself before 18th-century drainage works altered the landscape. Today, visitors can still see the remains of a remarkable 132-metre stone causeway that provided the only approach to the castle from the south, built at a time when the waters of Lough Gur would have surrounded much of the structure.

The causeway originally connected two castellated gatehouses; one at the southern end, now completely levelled and possibly beneath a modern bungalow, and another projecting from Black Castle’s curtain wall. Thomas Dineley’s 1680 drawing shows both gatehouses intact, describing the causeway as “The long bridge wth Drawbridges”, suggesting a sophisticated defensive system that could cut off access at multiple points. The southern gatehouse’s foundations, measuring 7.1 metres square according to early 19th-century accounts, protected the entrance to this vital crossing. Roughly halfway along the causeway, a deep drainage ditch now cuts east to west; this 440-metre channel was excavated by Henry Baylee of nearby Lough Gur Castle during his ambitious 18th-century attempt to lower the lake’s water levels.

The castle’s origins remain somewhat mysterious, with local historian Lewis attributing its construction to John Fitzgerald in the 14th century, whilst Lynch suggests an earlier 13th-century date, possibly built by Geoffrey de Marisco who received the manor of Knockainey in 1212. Whatever its precise origins, Black Castle formed part of a network of fortifications around Lough Gur, with Bourchier’s Castle visible 550 metres to the north and New Church 510 metres to the south-southwest. The site has been protected under a preservation order since 1976, recognising its importance as one of Ireland’s most evocative lakeside fortresses.