Built from roughly coursed limestone rubble with carefully dressed corner stones, the tower measures 9.1 metres north to south and 10.5 metres east to west, with walls over two metres thick. By the time of the Civil Survey in 1654-6, the castle was already described as being “out of repaire and is al wast by reason of their fastnes to the redd bogg of Ely”, with the Earl of Ormond listed as its proprietor. A nearby ringfort to the north hints at the site’s much longer history of occupation.

The tower’s defensive features reveal the careful thought that went into its construction. The main entrance, located off-centre on the eastern wall, consists of a round-arched doorway leading to a lobby that was once protected by an overhead murder-hole. From here, a mural staircase in the northeast corner provided access to the upper floors, though this is now blocked. The interior layout alternated between wooden ceilings on the ground and second floors and barrel vaults on the first and third floors, creating a robust structure capable of withstanding siege. Each level served different purposes: the ground floor features large segmental-arched embrasures in the north and south walls, whilst the first floor contains mural passages, and the second floor boasts a grand twin-light ogee-headed window that would have illuminated the main chamber.

What makes Ballybeg particularly interesting is its regional architectural style, especially the small barrel-vaulted chambers at second-floor level built into the east and west walls. These extend upward in small towers that project beyond the wall-walk on the north and south sides; a design typical of tower houses in this part of Ireland. Practical considerations weren’t forgotten either, with garderobe chambers tucked into the northwest angle of the tower, their outlet still visible at the base of the western wall. The ghost of a later addition can be seen on the eastern wall, where the crease line of a two-storey seventeenth-century house remains visible, showing how the castle was adapted and reused even as it fell into decline. Today, no traces of a protective bawn wall survive around the tower, leaving it to stand solitary against the Tipperary sky.