What makes this tiny landmass particularly intriguing is the deep, narrow fosse that separates it from the mainland; a defensive water channel that hints at its strategic importance in centuries past. Historian Davies, writing in 1947, proposed that this isolated spot once held a late 16th-century O’Reilly castle, which may have replaced an even earlier 15th-century fortification.

The O’Reillys were one of the most powerful Gaelic families in County Cavan, and their choice of this naturally defended island location would have been no accident. The surrounding waters and deliberately cut channel would have made any assault extremely difficult, whilst providing the castle’s inhabitants with clear views across the lough to spot approaching threats. Such island strongholds were common amongst Irish chieftains during the medieval period, combining practical defence with a powerful statement of authority over the surrounding landscape.

Today, visitors to Rahardrum will find no visible traces of these once-formidable structures above ground. The castle had already vanished by the time of the Down Survey in 1656-8, as it doesn’t appear on the contemporary map of Castlerahan barony. Whether it was destroyed during the tumultuous wars of the 17th century or simply abandoned and left to decay, the island now keeps its secrets beneath the soil, a silent witness to the centuries of conflict and change that shaped medieval Cavan.