Rising 18 metres high at its eastern end, this unusual fortification features a distinctive prow-like projection that houses a spiral staircase, with vaulted rooms arranged behind it. The castle's unique pentagonal design, measuring 16.5 metres at its greatest length and 10.4 metres wide, sets it apart from typical Irish tower houses. Built with walls 2.3 metres thick, the structure originally contained five storeys, with the lowest level featuring vaulted ceilings and the second floor boasting a fireplace on its north wall.

The castle's history stretches back centuries, with the Troy family establishing their presence here as early as 1197 when H. Troy served as Limerick's first provost. By 1583, the property had passed to W. Bourke, and later to the MacKeoughs, who held it until the mid-17th century. The 1654-56 Civil Survey recorded that Theobald, Lord Baron of Brittas, owned what was already described as a 'ruined castle' along with an orchard and an eel weir; a reminder that the Shannon's waters provided both defence and sustenance. Following various changes of ownership, including a grant to the Duke of York in 1666, the castle was eventually sold to the Hollow Blades Company in 1703.

Today, whilst the east and north walls remain relatively intact, the western and southern sections have largely crumbled away, with only fragments of the original structure surviving. The southeastern corner preserves interesting defensive features, including an angle-loop at second storey level and corbels that once supported a bartizan chamber at the fourth floor. Another bartizan chamber existed at the northwestern corner, demonstrating the castle's sophisticated defensive architecture. Despite its ruined state, Castle Troy remains an intriguing example of late medieval Irish fortification, its unusual five-sided plan and riverside location marking it as a significant architectural curiosity along the Shannon.