Following the sale of the house and the dispersal of the contents, 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 Castletowns 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. Gradually the Georgian Society, and after 1979 the Castletown Foundation, began to acquire Irish furniture and paintings for the house, through gifts and loans. The restoration of the interior of the house to its eighteenth century grandeur began at this time, with the re-decoration of the green silk room, in 1985, a notable achievement. The house continued to remain open, and the restoration process continued to proceed, still mainly funded by private munificence and visitor revenue.
In 1994, however the house was taken over by the Irish government and it is now under the management of the Office of Public Works. The Castletown Foundation continues to have an advisory role regarding the interior and still owns most of the original contents. Since 1994 the conservation process has continued at a faster pace. Major structural work was carried out, particularly on the roof, but also inside the house in the mid 1990s. Since then further conservation work has been ongoing both on the internal fabric of the house and on the contents.