Built c.1722 for the speaker of the Irish House of Commons, William Conolly (1622- 1729) the designs of a number of important architects were used, notably Alessandro Gailiei, Sir Edward Lovett Pearce and later Sir William Chambers.
The entire estate was sold by the Conolly-Carew family in 1965 to a property developer and in 1967 the house and some parkland were purchased by the Hon. Desmond Guinness. Both Mr Guinness and subsequently the Castletown Foundation, who acquired the house in 1979, devoted considerable effort and resources to maintaining the house and restoring the principal rooms to a high standard. Castletown was transferred to state care on 1 January 1994.