Nenagh Castle, in County Tipperary, was built by Theobald Walter (first of the Butlers of Ormond) around 1200. The castle boasts the finest cylindrical keep in Ireland. Like most keeps, it formed part of the perimeter of the fortress, being incorporated in the curtain walls surrounding a rather small, five-sided courtyard. These walls have now almost disappeared, but fragments remain. There were originally four flanking towers, one on each side of the entrance gateway to the south, the others at the east and west angles of the pentagon; the great keep, "Nenagh Round", occupied the northern angle.
Built of limestone rubble, irregularly coursed, and measuring about 55 feet in external diameter at the base, it rises now to a height of about a hundred feet. The topmost quarter, however, is modern (circa 1860), the original height to the wall-walk being about 75 feet. Above this there rose, of course, the crenellated parapets. There were four storeys, including a basement which was accessible, originally, only from the entrance storey above. At the base, the walls are 16 feet thick, and at the top, just 11 feet; the diminishing thickness is accounted for by the inward batter of the walls and the offset or setting back, at each floor level, which bore the timber work of the floors.
The top of the existing tower was probably quite uneven, and so it was raised and dressed off with a new parapet wall. The architecture is gracefully decorative, and includes a series of clerestory windows beneath a corbelled parapet wall ornamented with traditional stepped merlons.