Geroid Castle, Knockfennell, Co. Limerick

The island itself tells a fascinating story of changing water levels and hidden history; when the lake was drained by 1.5 metres around 1839, what had been a tiny 30-metre islet suddenly expanded to reveal a much larger landmass measuring 170 metres north to south. This dramatic transformation exposed not just more land, but an enormous accumulation of bones, with more than a hundred cart-loads removed and sold to dealers, hinting at the island's long and possibly grim past.

The castle ruins consist of a circular stone platform about 40 metres across, topped with the remains of a mortared wall that follows the circumference in straight sections, creating a polygonal enclosure. Though the structure is now largely collapsed and overgrown, with only traces of sunken rooms and a few surviving quoin stones, historical accounts suggest this was once a formidable fortification. In 1599, during the Desmond Rebellions, the Earl of Desmond seized the island from Queen Elizabeth's forces, and by 1600, Sir George Carew reported it housed John Fitz Thomas with at least 200 men, describing it as "a place of exceeding strength" surrounded by deep water.

Local legend adds another layer to the island's mystique, claiming it as the domain of Garret Fitzgerald, the rebel Earl of Desmond, whose ghost is doomed to ride across the lake's surface on a horse with silver shoes once every seven years until the shoes wear away and he can return to life. Archaeological excavations have revealed that human activity here stretches back much further than any medieval castle; significant Neolithic and Bronze Age settlement sites have been uncovered both on the island and the nearby Knockadoon peninsula, along with a causeway connecting the island to the shore. Today, visitors can still make out the circular platform that forms the castle's core, a testament to centuries of human occupation on this small but historically rich island.