The site has ancient roots; a bridge crossed the Shannon here from 1140 AD, making it a strategic location that Walter de Lacy recognised when he began constructing a castle at Ath Liagh in 1220. His successor, Geoffry Mares, completed this early fortification in 1227, though it was likely built of earth and timber rather than stone. The tower house that visitors see today came later, replacing or supplementing these earlier defences.

The castle’s documented history reveals its changing fortunes through the centuries. By 1571, it had come into the possession of the abbot of Cloontuskert, who controlled both the castle, then known as Boalalege or Ballyleague, and six cabins in the area. However, this ecclesiastical ownership proved short lived; by 1596, Irriell O’Farrel had lost the castle to Irish rebels during the tumultuous period of resistance to English rule. The Down Survey barony map of 1655 to 1656 shows Ballyleague castle still standing, marking its continued importance to the region even as political control shifted.

Today, the rectangular tower house remains partially intact, measuring approximately 8.8 metres north to south and 5.6 metres east to west internally. Though much of the structure has been robbed of its stone over the centuries, visitors can still trace its defensive features: horizontal gun loops pierce the walls at multiple levels, whilst the remains of fireplaces and windows hint at domestic arrangements. The ground floor entrance once led through a projecting tower on the eastern wall, which likely housed the stairs, whilst the upper floors featured round headed windows and additional gun loops. Despite its ruined state, the surviving walls reach to the second floor beneath the north to south vault, offering a tangible connection to centuries of Irish castle building and the strategic importance of this Shannon crossing point.