The castle’s documented history stretches back to 1583, when it was held by W. Bourke and known by various names including “Castellano Trylan” and “Nestermoy”. The fortification changed hands multiple times over the following centuries; Dominick Roche owned it in 1607, H. Holcroft was granted the castle and bawn in 1619, and by 1623 Jordan Roche held the property along with fifteen acres of orchard. The estate was eventually granted to the Duke of York and sold in 1703 to the Hollow Blades Company.

The tower itself is an impressive structure, originally measuring approximately 8.4 metres by 6.6 metres, with walls rising to about 15 metres in height and 1.7 metres thick. Historical accounts from 1840 describe the castle as having five storeys with numerous windows and apertures, though by that time the west wall had already collapsed. Local tradition maintains that a square tower once stood at the southwest corner, which fell around 1800, taking the west side with it. Despite these losses, much of the structure remained intact well into the 19th century, featuring large windows, high gables, and chimneys typical of late medieval Irish tower houses.

Local folklore adds an intriguing, if unlikely, dimension to the castle’s story, with tradition claiming that King William III resided here during the Siege of Limerick in 1690-91. The castle’s strategic position, bounded by the River Griodane to the west and the Shannon to the north, would have made it a valuable stronghold during various conflicts. In recognition of its historical significance, Newcastle Clanwilliam received a Temporary Preservation Order in 1984, ensuring its protection as an important monument to Ireland’s medieval heritage.