Fahan Castle, granted to Sir John O’Doherty in the late 16th century, met its demise around 1600 when it was partially dismantled. By 1601, English observers noted that the castle, which stood opposite Inch Island in O’Doherty’s territory, had been “broken down since our arrival”, though Bishop O’Gallagher still resided nearby. Historical records describe the structure as a square tower surrounded by a circular bawn, a typical defensive arrangement for Irish tower houses of the period.

The exact location of the castle remained a mystery until intriguing evidence emerged during building work in the 1940s. When extending Castletown House, located about 400 metres south of Castletown Cottage, workers discovered vaults or arches adjoining the north wall of the house. Amongst the rubble, they found the head of a single light ogee headed window, punch dressed in 16th century style with sunken spandrels and a bar hole at the top of the block, suggesting it may have been repurposed as a door jamb. This architectural fragment, along with the vaulted structures which could have been the remains of a barrel vaulted ground floor, strongly suggests that Castletown House sits on or near the site of the medieval castle.

Archaeological testing in 2007 ahead of agricultural development at Magherabeg revealed little more than modern pottery sherds, water services, and ceramic land drains, finding no significant archaeological remains in the immediate area. However, the medieval window head discovered decades earlier remains visible in the farmyard adjacent to Castletown House, alongside a bullaun stone, serving as tangible reminders of the castle that once controlled this strategic position overlooking the waters toward Inch Island.