Originally accessible by land, the castle now stands surrounded by water following the Lee Valley Hydro-electric Scheme in the 1950s, with only a road bridge connecting it to the eastern shore. The structure consists of a three-storey rectangular tower that has been extensively altered over the centuries, with an annexe tower built against its northern wall. A vertical line of quoins on the south face reveals the original extent of the tower, which was extended approximately four metres eastward at some point in its history.

According to tradition, Cormac Mac Carthy built the initial tower in 1455, though the castle continued to evolve well into the 18th century. The eastern extension likely dates from the 16th century, whilst the annexe tower may have been added in the late 16th or early 17th century. The main tower’s interior spans roughly 15.8 metres east to west and 5.6 metres north to south, containing numerous blocked windows, gun loops, and fireplaces that hint at its defensive past. The second floor features particularly interesting details, including three windows along the south wall with segmental vaults, one retaining an elegant ogee-headed light. A small box machicolation supported by two corbels juts from the south wall near the southeast corner, a reminder of the castle’s military purpose.

The annexe tower adds another layer of complexity to the structure, housing a spiral staircase (though all steps have since been removed) and small chambers connected to the main tower through quarried passages. Its ground floor once featured an external door with a pointed arch, approached by forestairs, though only fragments of this entrance survive. The castle gained a sombre historical footnote during the 1650 siege when Bishop Mac Egan was hanged here, an event now commemorated by a modern stone plaque. Before flooding, the western part of the tower even served as a handball alley, demonstrating how this medieval fortress adapted to community needs across the centuries.