Built in the 1620s or 30s by Margaret Grant, widow of Gibbon FitzGibbon, this substantial fortification measures 16 metres long by 10.2 metres wide. The Ordnance Survey’s meticulous 1840 description reveals a structure that combined defensive features with comfortable living spaces; thick limestone walls reinforced with brick, winding stone staircases ascending 8.5 metres, and plastered interiors that suggest a level of domestic refinement unusual for such buildings.

The castle’s architectural details tell a story of evolving security needs and changing tastes. Originally accessed through a western doorway built with chiselled limestone and protected by a single gunloop, the entrance was later relocated to the north side, complete with its own murder hole. The interior featured a clever arrangement of spaces, with vaulted rooms flanking the entrance passage and a broken spiral staircase in the northeast corner. Multiple fireplaces throughout the building, including a fine example on the second storey’s north wall and another on the third storey’s south wall, indicate that comfort wasn’t entirely sacrificed for defence. The windows, described as quadrangular with some narrow round examples, would have provided both light and defensive positions.

The historical record places Dunmoon Castle at the heart of a thriving Irish settlement. The 1654 Down Survey noted it as “a Faire Castle and a good Irish Towne”, whilst the Civil Survey of the same period recorded that the property included not just the castle but also a bawn, a mill, three thatched houses with stone chimneys, and about 22 small cabins. The estate’s ownership can be traced through various legal documents from 1300 onwards, with the FitzGibbon family, particularly the branch known as the White Knights, maintaining control through much of its history until it was sold to Robert Oliver following the Cromwellian conquest.