The main building is a Norman Revivalist castle. It faces south, and commands an unbroken view of some thirty miles towards the Galtee Mountains. It was designed by William Bardwell, an English architect, who evidently intended Glenstal to look like a twelfth-century castle, for when he inscribed his name on the turret, Bardwell me fecit (‘Bardwell made me’), he added the date 1839, but cut in such a way as to look like 1139.

The castle was built for the Barrington family in the 1830s. Following the death of their daughter in an ambush during the Irish War of Independence, the last owners, Sir Charles and Lady Mary Rose Barrington, decided to return to live on their English estate. The property was bought by a local priest, who invited the monks of Maredsous Abbey in Belgium to settle in Glenstal. Monastic life at Glenstal began formally on 18 December 1927.