This fine Norman Keep was built c. 1200 by Theobald Walter, 1st Baron Butler and completed by his son Theobald le Botiller c1220. The circular keep is over thirty metres high and its base has a diameter of sixteen metres. It is one of the finest of its kind in Ireland. The crown of crenellations and ring of clerestory windows were added at the instigation of Bishop Michael Flannery in 1861.
The intention was that the keep would become the Bell tower of a Pugin-designed cathedral that was never built. Though not true to historic character, these additions have ensured the iconic status of the keep which ensures that it features on the logos of many local clubs and businesses including Nenagh Town Council.
The Castle and grounds were extensive renovated between 2009 and 2013. This project is aimed to position the castle as the main tourist attraction in the area. It is now open to the public.