A 16th-century tower house still stands, and built directly onto the western wall of its bawn is an early 17th-century house, the two structures sitting just metres apart in what was once a carefully organised defensible compound. A bawn, for those unfamiliar with the term, is the fortified enclosure wall that typically surrounded an Irish tower house, often serving as a yard for livestock and household activity. Here, the bawn became the backbone around which successive generations kept building.

The house itself dates to around 1600 to 1620 and was described by the architectural historian Mike Salter as having a south end wall, a wing extending east, and most of a wing projecting west toward the older tower. A small back court was enclosed to the west, fitted with its own doorway and a corner bartizan, a small projecting turret corbelled out from the wall. To the east, a bawn roughly 20 metres square was laid out. The ffox family, who appear to have held Ballygrennan at this period, lost the property in 1621, though they had recovered it by 1657. By that point a much larger outer bawn, 38 metres long by 26 metres wide, had been added on the north side, complete with a north-facing gateway and corbels for another corner bartizan. When the Evans family acquired the castle in the 1660s, they appear to have inserted an outbuilding into the west end of this outer bawn, fireplaces on the upper level suggesting it served as a stable below with quarters for grooms and servants above.

The most vivid account of what Ballygrennan once looked like comes not from an architectural survey but from a soldier's journal. On 11 September 1690, John Stevens recorded marching back toward Bruff and passing a great house belonging to one Evans, large, built in the manner of a castle with stone walls and battlements, and already burnt by his own Jacobite army. Stevens noted the surrounding country approvingly, commenting on good land, corn, cattle, cabbages, and especially the vast fields of potatoes that sustained both soldiers and local people alike. The ruin visible today carries the scorch marks of that episode, at least in its history if not always in its fabric. The site sits close to the tower house, and both are best approached with an awareness that what remains is fragmentary; walls, angles of rooms, and traces of a compound that once functioned as a working, inhabited estate.