The Mac Dermot family, descendants of Gilchrist Clerig Mac Dermot who died in 1325, established themselves at Leamgire during the 14th century. By 1549, Rory Mac Dermot, who ruled from 1549 to 1568, assisted the sons of Donnchadh Dubh in constructing or renovating the castle. The fortress remained in Irish hands through the late 16th century, with Owen Grana, Donnchadh Dubh’s grandson, holding ownership in both 1585 and 1617.

The early 17th century brought significant change to Castletown, reflecting the broader transformation of Irish landownership during the plantation period. In 1622, two of Owen Gran’s sons sold the castle and its fortified enclosure, or bawn, to Josiah Lambert, an English settler who had also acquired nearby Canbo. This transfer marked the end of Mac Dermot control over this particular stronghold, though the family maintained influence elsewhere in Roscommon.

Today, virtually nothing remains visible of this once important fortress. The 1914 Ordnance Survey six-inch map marked its location on a low ridge running north to south, noting that only foundations about 1.8 metres thick survived at that time. Even these have since disappeared from view, though a single dressed stone incorporated into a nearby farmhouse gable serves as a lonely reminder of the castle’s existence. A rath, or ring fort, lies approximately 130 metres to the northeast, suggesting this area held strategic importance long before the Mac Dermots built their tower house.