This rectangular tower, measuring 14.65 metres east to west and 9.7 metres north to south, features an unusual design element: triangular projections or spurs at its southwest and northeast corners. These distinctive defensive features, which taper to blunt points extending about 6.4 metres from the tower walls, contain triangular chambers at first-floor level, each equipped with four gun loops strategically positioned to cover the tower’s vulnerable corners and provide outward defence. Similar spurs can be found at Carrignacurra Castle and Castle Hyde in North Cork, suggesting this was a regional defensive innovation of the period.

The tower originally stood at least five storeys high, though vandals destroyed much of the structure around 1864 when they blew up the southeast corner, taking with it most of the eastern wall and the eastern half of the southern wall. What remains reveals sophisticated internal planning: the ground floor features a main chamber measuring 7.4 by 5.4 metres, lit by splayed window embrasures with segmental vaults, including one with an ogee-headed light in the north wall. The first floor, covered by a rounded vault running east to west, provided access to the corner projections through doorways, whilst the upper floors contained fireplaces in the north wall and what appear to have been mural chambers built into the thickness of the eastern wall, though only their northern end walls survive today.

Dating to around 1584, when ownership transferred to the Mac Sweeneys, Mashanaglass Castle represents the military architectural developments of its era, when gun loops were becoming essential defensive features in Irish tower houses. The presence of ogee-headed windows, particularly the double-light example at second-floor level, adds a touch of decorative sophistication to what was primarily a defensive structure. A well, marked as ‘Castle Well’ on the 1938 Ordnance Survey map, sits immediately northwest of the castle, though it’s now covered and serves as a pump house, a modest reminder of the castle’s once self-sufficient domestic arrangements.