Early history On William Conolly's death in 1729, his widow
Katherine (née Conyngham) continued living in the house and hosting extravagant entertainments there until her own death in 1752. The Castletown estate then passed to Conolly's nephew
William James Conolly, MP for
Ballyshannon. On William James' death in 1754 it passed to his son
Tom Conolly whose wife,
Lady Louisa (great-granddaughter of
Charles II of England and
Louise de Keroualle), finished the interior decoration during the 1760s and 1770s. Lady Louisa had grown up in Carton House, within a demesne to the north-east of Castletown house. Much of the work on the interior was carried out to designs of
William Chambers. Lady Louisa also ordered extensive work on the grounds; the drainage scheme through the woodland is ingenious, creating dry paths for walking on land that is below the watertable (the ha-ha fence is part of this intricate network). The paths through the forest she set out are still in walking condition although, due to anti-joyrider measures, several of the culverts have broken and the pathways are again subject to flooding. When Tom Conolly died in 1803, he left the estate to his wife, who on her own death in 1821 bequeathed it to her great-nephew,
Edward Michael Pakenham, later the MP for
Donegal, on condition he adopted the surname of Conolly. He was succeeded by his eldest son, another
Tom Conolly, the eccentric MP for Donegal from 1849 to 1876. Tom's eldest son Thomas inherited Castletown, but after he was killed in 1900 the estate passed to his brother Major Edward Conolly. The Conolly family continued to reside in their ancestral home, later bearing the compound surname Connolly-Carew and the title
Baron Carew, until 1965. That year,
the 6th Baron Carew sold Castletown House—along with its contents and 580 acres of land—for £166,000. The purchasers were Julian de Lisle, a London-based property owner, businessman, and cousin of the Guinness family, and Major James Wilson, Master of the Kildare Hunt and also a cousin of de Lisle. Portions of the estate have since been sold and developed, although some original features have been preserved within the new constructions.
Desmond Guinness and the Castletown Foundation The house was bought in 1967 by
Mariga and
Desmond Guinness for £93,000 to save it from vandalism, became the flagship of the
Irish Georgian Society, and was eventually handed over to the newly established
Castletown Foundation. The estate was sold on in parts, with held as curtilage around the house, some small quantities retained privately by Desmond Guinness, the forested
Crodaun Woods part sold to what became
Coillte and around acquired over time by
Kildare County Council. Most of the core estate remains as woodland or green space but a large fraction in the south west was developed as
Castletown Estate, controversially approaching the main house closely, and taking in the Walled Garden and the orchard (the wall of which remains), which were lost. However, some other features, such as the Gazebo, the Steward's House and a mock temple, were retained, after negotiations between developer Janus Securities and the Irish Georgian Society.
Modern times The charitable Castletown Foundation struggled with the financial and operational demands of maintenance and ongoing restoration, and in 1994, following extensive negotiations between the foundation and the State, Castletown House, with its 120-acre residual demesne, was transferred to the Office of Public Works (OPW). In 2024, the OPW acquired
Donaghcumper House and ancillary lands from
Kildare County Council. Located south of the river Liffey, opposite Castletown House, this purchase ensured the protection of the historic vistas to and from Castletown House. In November 2025, the Irish State purchased the remainder of the Castletown Demesne, bringing the total area of lands around the House in State ownership to 475 acres. The OPW continued the programme of restoration, of both house and lands. Across the broader former estate, and despite the protection of the house and some estate features, at least one of the outlying features,
The Gazebo, was partly demolished, without planning permission, in late 2007. An enforcement notice was served on the developer concerned, and further action was expected. In 2012, work began to restore the lake on the lawn between Castletown House and the Liffey, and this was followed in 2016 by work on the pleasure grounds behind and to the west of the house. Following some years of dispute over access and visitor parking, after the sale of parts of the demesne, the Office of Public Works purchased some 235 acres of land for €11.25 million, including the access from the M4 motorway. This was added to the existing 227 acres of the Castletown Demesne already in State ownership and OPW management. This purchase brought to an end the long running and bitter dispute over public and vehicular access to the estate. On 22 December 2025 full public access to the Castletown estate was restored. ==Access and rights of way==