Following the sale of the house in 1965 and the dispersal of the contents a year later, the house was left vacant and subjected to vandalism.
Lead was stripped from the roof, and windows were broken. Miraculously, the remaining fittings, including all the original chimney-pieces, remained. An inglorious end to Castletown’s proud history seemed inevitable. However, in 1967, the Hon. Desmond Guinness purchased the house and 120 acres of land to save it for posterity. Castletown became the flagship project of the Irish Georgian Society, which had been re-established by Desmond and Mariga Guinness in 1958. In 1967 it became the first house in Leinster to be opened to the public.
The restoration of Castletown began under the aegis of the Irish Georgian Society.
Their first major task was to acquire furniture and paintings for the house. Many of the original furnishings had been secured for the house at the auction by Desmond Guinness. Acquisitions as well as restoration work were funded by the generosity of private benefactors and were realised with the help of many enthusiastic volunteers. In 1979, the Castletown Foundation was established to take over the ownership and management of the house, but the upkeep of the house presented an uphill struggle. Following negotiations, the Office of Public Works (OPW) accepted the property on behalf of the State in 1994.
Starting that year, the OPW launched an extensive and ongoing conservation programme at Castletown.
The first phase of works included much needed external repairs to the fabric of the main block and stonework. On the exterior, the main block was covered with a protective temporary roof and the entire roof and parapet were dismantled and repaired. The second phase, completed in autumn 1998, addressed issues which affected the internal environment and included rewiring as well as the upgrading of the security and fire alarm systems. The third phase of works focused on structural issues and the repair of internal finishes in the main block. The ceiling of the Long Gallery was strengthened and the plaster-work conserved before the house reopened to the public in 1999.
Today, visitors to Castletown can enjoy a fine collection of Irish decorative arts within a setting that is famed both for its architectural significance and for the beauty of its natural environment. Great care has been taken not only in the conservation of the mansion but also in the restoration of the landscaped parklands, waterways and follies around it, which date back to the eighteenth century. An example of ongoing conservation works in the house is the Red Drawing Room with its nineteenth-century silk wall hangings. In 2016, the Pleasure Grounds stretching behind the house and to the west were restored, creating a quiet pleasurable retreat for visitors.