Castleboy, Portaferry Road, Castleboy, Cloghy, Newtownards, Co. Down, BT22 1HP

The ruin at Castleboy is a fragment of a former tower house, likely of 15th-century origin, situated on the edge of a field off the Portaferry Road in the townland of Castleboy, near Newtownards. The surviving structure represents the southeast corner of the tower and may have been a turret that was once part of a rectangular tower with a similar turret at its northeast corner. It stands approximately 10.5 meters tall and features a small opening at the upper level. The cornerstones have been removed, and the remaining wall elements are now overgrown. The site is located around 90 meters southeast of the medieval church of the Order of St. John of Jerusalem.

The name "Castleboy," meaning "the yellow castle," may indicate the original owner was a Gaelic or Gaelicised lord. While maps show the castle's location in the late 16th and early 17th centuries, the earliest written reference to its inhabitation is in a Gaelic poem dating to around 1617. By the late 17th century, the castle had been abandoned and was described as a "great ruinous pyle." By the 1830s, only a "ruined fragment" remained, as recorded in the Ordnance Survey Memoirs. The structure is considered a monument.