Though the Ordnance Survey maps mistakenly label these ruins as ‘Plary Monastery’, historical records and architectural evidence tell a different story. The actual monastery, dedicated to St. Mary, stood about 150 to 200 metres to the east, on the opposite side of the river, though its precise location has been lost to time. The 1656 Down Survey map clearly shows both structures; a ‘ruinous abby’ within a D-shaped enclosure east of the river, and a large building or castle to the west, which corresponds with these surviving ruins. Local tradition in 1837 insisted the tower was part of the monastery, but even the antiquarian John O’Donovan expressed doubts, noting it had ‘all the appearances of a strong military tower’.

The structure itself is a fascinating example of late medieval architecture, likely dating to the 15th or 16th century based on construction techniques visible in the surviving masonry. The circular tower, with walls 2.3 metres thick and an internal diameter of 3.8 metres, originally rose several storeys but now stands only two floors high. Entry was through a doorway on the south-southeast side, complete with a drawbar slot for security, leading to a ground floor chamber covered by a stone vaulted roof. A mural staircase, still showing traces of wicker centring used during construction, winds up through the walls to access the upper floors. The ground floor features two defensive arrow slits set at different heights; one with a pointed arch embrasure in the north wall, another with a flat-headed embrasure to the west. Attached to the tower’s eastern face are the remnants of an adjoining single-storey building, with grass-covered wall footings suggesting a possible bawn extending roughly 22 metres east to west.

The castle’s documented history begins properly in 1606 when King James I granted John Wakeman ‘the castle of Balleemore-Loughsewdie’ along with four carucates of adjoining land, fishing rights, mills, weirs, a Wednesday market, and an October fair. During the Williamite siege of Ballymore in 1690, it was described simply as ‘an old Castle nigh a quarter of a Mile to the South-West of the Fort’, and the structure was badly damaged during this conflict. By 1691, it appeared on a sketch plan as ‘The round Castle’, cementing its military rather than religious identity. Though its builder remains unknown, whether an Irish lord or perhaps even a religious house seeking defence, the tower stands as a reminder of Ballymore’s strategic importance through the centuries, offering commanding views across the surrounding landscape from its position atop a low rise.