This tower house, once belonging to Donagh O’Brien of Ballina in 1640, now exists as little more than a single corner; its southeastern angle rising four storeys from a small island just 70 metres from the shoreline. Built from sandstone rubble with carefully placed quoins, the structure once commanded this sheltered inlet, and historical records from the 1650s describe it alongside a thatched tenement that stood opposite.

Today, visitors can still trace the castle’s original features amongst the ruins. The surviving walls, measuring over two metres thick, contain remnants of a stone vault at ground level and a mural staircase within the eastern wall that once provided access to the upper floors. A doorway in the south wall opened onto these stairs, whilst deep window embrasures, now mostly collapsed, would have lit the interior chambers. The castle’s construction shows skilled craftsmanship; well-grouted stonework bound with lime and sand mortar has endured despite the structure’s ruinous state, though large chunks of fallen masonry now litter the site around the remaining walls.

Archaeological surveys and historical accounts paint a picture of gradual decay. An 1840 description noted the castle stood roughly 18 metres high, with the small island beneath covered in rounded stones, possibly indicating artificial construction typical of crannógs. By then, massive brown sandstone blocks had already tumbled from the western side, and though some modern concrete repairs have been attempted at second floor level, the castle continues its slow collapse. The surviving southeastern corner, with walls extending 5.8 metres on the east side and 4.7 metres to the south, offers a glimpse into the defensive architecture that once protected this strategic position on Lough Derg, giving the surrounding townland of Castlelough its name.