McDermott's Castle, Medieval castle on Castle Island in Lough Key, County Roscommon, Ireland

McDermott's Castle stands on Castle Island, a 0.23-hectare island in the southeast corner of Lough Key. Its main structure is a late medieval tower house with an arrow slot and medieval windows on the western wall, flanked by early 19th-century additions by architect John Nash. A stone enclosure wall surrounds the island, and the tower's crenellations date largely to the 19th century.

The original castle dates from the 12th century but was destroyed by lightning in 1184. The MacDermott clan rebuilt fortifications on the same site afterward.

The castle's Irish name, Carraig Mhic Dhiarmada, ties it to the Mac Diarmada, rulers of Magh Luirg from the 10th to 16th centuries. A poem for Tomaltach an Einigh mac Diarmata, King of Moylurg 1421–58, recounts the Hag of Lough Key, who stayed on the Rock a full year and bound the McDermotts to perpetual hospitality. After the family lost the island in 1586, Eochaidh Ó hÉoghusa wrote a poem lamenting the castle's emptiness.

The castle stands on Castle Island, a 0.23-hectare island in the southeast corner of Lough Key, County Roscommon. It is a protected national monument and the island remains in private Irish ownership, so the structure is seen from the water rather than entered. The enclosure wall was modified at its northern end to open a view from the main window inserted into the tower.

In 2019 excavations on the island found silver pins, a gaming piece and large collections of butchered cattle, boar and sheep, and showed the island itself is a multi-period fortification with early medieval elements. A two- to three-metre-thick enclosure wall predating the standing one lay a metre and a half down.