Castletown House is a Palladian country house built in 1722 for William Connolly, the Speaker of the Irish House of Commons. It formed the centre piece of a 550 acre (220 ha) estate. The house was bought in 1967 by Desmond Guinness for £93,000 to save it from vandalism and it became the flagship of the Irish Georgian Society. In 1994, the house was transferred to state ownership and is now managed by the Office of Public Works.
The Portland stone staircase at Castletown is one of the largest cantilevered staircases in Ireland. It was built in 1759 under the direction of the master builder Simon Vierpyl (c.1725–1811). The solid brass balustrade was installed by Anthony King, later Lord Mayor of Dublin. The opulent rococo plasterwork was created by the Swiss-Italian stuccadore Filippo Lafranchini, who, with his older brother Paolo, had worked at Carton and Leinster House for Lady Lousia’s brother-in-law, the first Duke of Leinster, as well as at Russborough in Co. Wicklow. Shells, cornucopias, dragons and masks feature in the light-hearted decoration which represents the final development of the Lafranchini style.