This rectangular limestone fortress, measuring 9 metres east to west and 7 metres north to south, rises approximately 15 metres high; its walls built from undressed limestone blocks laid in careful courses. The McGeoghegan family tower features a distinctive external base batter about 5 metres high, with dressed quoin stones marking the corners, though time has loosened several which now lie scattered at the building’s base. A small stream runs north to south roughly 18 metres to the east, where the remains of an old corn mill complex, marked on the 1910 Ordnance Survey map, have since been incorporated into a modern farmyard.

The castle’s current entrance sits at the western end of the south wall, though this plain rectangular doorway appears to be a later addition; the original entrance was likely the now blocked arched opening at the northern end of the east wall. Inside, a thick cross wall running north to south divides the eastern portion of the interior, creating a narrow chamber accessed through an arched opening that still shows traces of its original wicker centering. A spiral staircase tucked into the southeast corner once provided access to the upper floors, though it’s currently blocked at ground level and used for storing building materials. The first floor retains its stone vault and features a fireplace built into the south wall, whilst the third floor also contains a fireplace on the south side and a garderobe in the northwest corner, its chute opening visible from outside.

Despite centuries of exposure to Irish weather, the tower remains in remarkably good condition, with traces of plaster still visible on some internal walls and various architectural details intact. Plain rectangular window openings pierce all four walls, complemented by narrow arrow loops at the upper levels, some still retaining dressed stones on the external faces of their slit opes. Two gothic lancet windows, originally part of this structure, were removed and later reset in the tower at nearby Tyrrelspass. Though the outbuildings depicted on historical maps have long since vanished without trace, Syonan Castle continues to command its position on the ridge, a testament to the medieval engineering and ambitions of the McGeoghegan clan who built it.