Hillsborough Castle, The Square, Hillsborough, County Down, BT26 6AG

Hillsborough Castle is a detached Neo-classical country house of two and three storeys, built on an L-plan, with the west wing constructed around 1780 and the east wing added around 1795. A Giant Ionic portico was added to the south garden front around 1835. Described by architectural historian C.E.B. Brett as "by far the largest and grandest house in north County Down", it was for one hundred and fifty years the seat of the Marquesses of Downshire, and is now in the ownership of the Northern Ireland Office. The house is laid out around a front courtyard on the west side of The Square, enclosed by a decorative gate screen, with extensive mature gardens to the south and west.

The roofs are hipped natural slate with lead ridges, set behind a parapet wall, with several tall sandstone ashlar chimneystacks with clay pots. Lead hoppers break through the parapet wall and lead downpipes serve the main elevations, with cast-iron downpipes to the remainder. The walling is red sandstone ashlar with a projecting plinth course and a moulded sandstone cornice to the lead-lined parapet wall. Window openings are square-headed with sandstone sills and timber sash windows, though some have been replaced with security windows.

The West Wing consists of a central two-storey block seven windows wide, with a pedimented breakfront three windows wide, flanked by a pair of single-bay projections — two-storey to the south and three-storey to the north. The north projection is extended by a further three-storey over basement north wing, two windows wide. Sandstone architrave surrounds with pediments on scrolled console brackets appear to the central opening, to both ground floor windows of the south projection, and to the ground floor window of the re-entrant bay of the north projection. Ground floor windows are 6/9 timber sash, except for the central opening which has timber French doors with a six-pane overlight opening onto four nosed steps. Security windows appear to the first floor, with 3/3 or 3/6 timber sash windows to the remainder. The central block opens onto a gravel terrace with nosed stone steps at either end leading to formal gardens.

The north wing has two painted gabled elevations to the north, each with a tall sandstone chimneystack, and a lower flat-roofed central connecting section. The front pile of the north wing is abutted by a single-storey over basement laundry wing with a hipped natural slate roof and 6/6 timber sash windows. The east elevation of the west wing is four windows wide and three storeys tall with rendered walling and timber sash windows, abutted by sandstone accretions extending from the east elevation of the east wing.

The symmetrical two-storey east elevation is five windows wide with a slight breakfront fronted by a tetrastyle Ionic portico, flanked by a pair of single-bay three-storey decorative projections. Security windows appear to the first floor, with a pair of large round-headed window openings to either end bay containing tripartite timber sash windows and a fanlight over. The portico comprises paired Ionic columns supporting a full entablature with a central square-headed door opening with pediments on scrolled console brackets and double-leaf flat-panelled timber doors. The door is flanked by a pair of 6/6 timber sash windows, and there are several wall-mounted coat-of-arms stone plaques and a cast-iron wall-mounted post box. The projections at either end have a breakfront bay to the upper floors on both elevations, supported on decorative console brackets, with French doors at ground level and 3/3 timber sash windows above.

The East Wing fronts onto the front courtyard with an informal composition to the north elevation, seven windows wide, with a two-storey projection at its east end abutted by a further two-storey over basement with attic block further east, this block fronting onto The Square. Extending from the south projection of the west wing is a flat-roofed single-storey gallery, five windows wide, terminated by a tetrastyle Ionic portico to an entrance breakfront. First floor windows are 3/3 timber sash and ground floor windows are 6/6 timber sash. The shallow portico has four Ionic columns supporting a plain entablature containing a square-headed door opening with architrave surround and double-leaf flat-panelled timber doors flanked by a pair of 2/1 timber sash windows. The east projection is three windows wide and three windows deep, abutted by a flat-roofed sandstone accretion to the east. The three-storey east block steps behind the east projection, is aligned with the front gate screen, and is abutted by a flat-roofed entrance porch with a replacement hardwood panelled door. Facing east, the easternmost block is three windows wide with a front garden enclosed by cast-iron railing and a matching pedestrian gate.

The symmetrical two-storey south garden front is fifteen windows wide, with a pedimented tetrastyle Giant Ionic order portico and a pair of shallow two-storey three-sided canted bays. The central section has a balustraded parapet wall, with a plain parapet wall to the remainder. First floor windows are 3/3 timber sash, with replacement security windows to the west end, and ground floor windows are 6/6 timber sash. The prostyle portico has a lead-lined roof and sits on a crepidoma-style platform of five nosed steps. The soffit of the portico has stepped coffering, with a tripartite central opening with a cornice supported on scrolled console brackets, and security-glazed French doors with sidelights opening onto three nosed steps. A further slender window opening appears to either side, with a small central window opening to the upper storey. The portico opens onto an elevated gravel terrace spanning the full south garden front, with a series of nosed stone steps descending to the stepped garden terraces. At the east end of the terrace is a Greek temple-style summer house facing west.

The house was damaged by fire in 1934 and has had a succession of internal reinstatement works carried out since, meaning the interior retains much of its original character despite these interventions. The exterior likewise retains much of its original fabric. The quality of the walling, steps, balustrading and piers enhances the principal house.

The setting is significant. The house is laid out around a front courtyard on the west side of The Square, enclosed by an elaborate gate screen. The formal west and south fronts are set on an elevated site with formal landscaped gardens on a gradient, enclosed to Dromore Road by a tall sandstone ashlar wall, and bounded to the rear of the houses on Main Street by the former stable yard. Sandstone low walling and balustrading define the elevated south terrace, with stone steps leading to the formal gardens. The unusual proximity of this country house to the town of Hillsborough gives the town a distinct focal point and creates an important group with the Courthouse and The Square. The demesne also includes the stable yard, formal and landscaped gardens, and a range of garden structures.

The history of the house is closely tied to the Hill family, who came to Ireland from the west country in the late 16th century. Moses Hill obtained extensive estates through the conquest of Irish chieftains and built a fortified house at Hill Hall in the early 1600s. His younger son Arthur Hill was the first to live at Hillsborough and reconstructed Hillsborough Fort after it was destroyed in the 1641 rebellion. The village was given borough status after the Restoration in 1660 and became the residence of the Hill family, who grew steadily in prominence. Trevor Hill became Baron Hill and Viscount Hillsborough in 1717. In the late 17th century Trevor Hill built a house close to the terrace of the present castle, described by Harris as "a noble large house built within the area of a regular fortification", though no drawings or plans survive. This house burned down in an accidental fire sometime in the late 1730s.

Wills Hill, the first Marquess of Downshire (1718–93), built a mansion house to the south-east of the present house in Large Park, the remains of which survive. Mrs Delaney's observations of 1758 suggest that Lord Hillsborough already had in mind the construction of a new mansion at that time, but an estate map of 1771 shows only a schematic representation of a terrace of houses on the western side of The Square. The present house appears unequivocally on an estate map surveyed in 1788 by W. Byers. Drawings by Brettingham, dated by the Public Record Office of Northern Ireland to around 1788, depict the west front as it stands today, and in an annotation to another drawing Brettingham describes himself as "the architect of the west range of the house itself", though the extent of his involvement in designing the remainder at that stage is not clear.

Wills Hill was Secretary of State for the Colonies during the American independence struggle, and Hillsborough was visited by Benjamin Franklin in October 1771. Unfortunately the two men disliked each other, Hill being unable to countenance American independence, and King George III later blamed the Marquess for the loss of America. Hill's plans for the town also included a new market house, the rebuilding of the Fort, and the rebuilding of the parish church.

The second Marquess, Arthur Hill (1753–1801), made additions and alterations to the house using the wealth of his heiress wife, again employing Brettingham, who added a library to the south-east of the original house and then a thirteen-bay south front around 1795. Numerous drawings survive showing Brettingham's designs both internally and externally, some clearly carried out and others apparently more speculative. A series of letters from William Forsyth, clerk of works during these improvements, to the Marquess of Downshire also survives. An estate map of around 1800 shows the house with its new south front, and the north wing that had appeared on the 1780 map now gone.

The third Marquess, also Arthur Hill (1788–1845), oversaw further changes. The main road to Moira ran across the south front at this time and by 1810 Lord Hillsborough was planning to re-route it to enhance the appearance of the mansion. The new road was opened in 1826 and the public road in front of the house, visible on the Ordnance Survey map of 1833, was closed between that date and 1839, when a plan recording the changes was made by Henry Murray. The third Marquess presided over alterations and additions by Thomas Duff in the late 1820s and by William Sands — who was resident in the town during the works and for the remainder of his life — in the 1840s. Letters from Thomas Duff to Lord Downshire survive from between 1828 and 1830, though the nature of the works referred to is unclear. Plans by Henry Murray of 1833 and 1839 show that between those dates the library was extended and given a giant portico, though Murray is not known as an architect and it is uncertain whether he designed these alterations.

At the time of the Townland Valuation (1828–40) the house was valued at £100, and a lengthy list of buildings and outbuildings with their dimensions appears in the fieldbook. The house is captioned as "Castle" on the first edition Ordnance Survey map of 1833.

William Sands, working with his relative James Sands, made several changes to the house and demesne in the 1840s, giving the house much of the appearance it retains today. The south front was extended to the east and a Giant Ionic portico was added; to achieve symmetry, a single bay was demolished to the west. The Parliamentary Gazetteer of 1846, while noting a criticism that the mansion might have been better placed farther from the public road, also observed that "the town acquires an almost aristocratic air from the proximity of the mansion and seems as if caressed between the lawn and the park." By the time of Griffith's Valuation (1856–64) the building, now described as a castle, was valued at £250. In 1867 a valuer noted that a new billiard room — a two-storey room of cut stone measuring approximately 20 by 17 feet — had been added, raising the valuation to £265.

The fifth and sixth Marquesses of Downshire had little connection with Hillsborough, preferring to live elsewhere. During the last quarter of the 19th century Lord Arthur Hill, younger brother of the fifth Marquess, lived at the castle, managing the estates and representing County Down in parliament. The sixth Marquess (1871–1918), who succeeded to the title in 1874 as a young child, was the largest landowner in the north of Ireland at the end of the 19th century, but at the beginning of the 20th century his estates began to be sold off under the Land Acts, and Lord Arthur retired to his London house. The house was subsequently let to Sir Thomas Dixon, son of Sir Daniel Dixon, a former Lord Mayor of Belfast. Sir Thomas served as High Sheriff of County Down in 1913 and occupied the castle from around 1910, leaving in 1919 when he purchased Wilmont near Dunmurry.

In 1922 the house was purchased by the Ministry of Works in London as a residence for the Governor of Northern Ireland, a newly created post following partition. Following three years of preparation, the Duke of Abercorn took up residence in 1925 in what was then renamed Government House. The Ministry of Works in London, later the Department of the Environment, retained responsibility for the fabric of the building until 1990, when ownership passed to the Northern Ireland Office. Following the fire of 1934 the house was refurbished internally under the supervision of Roland Ingleby Smith, and the gate screen was added to the market square entrance. Successive Governors — the Duke of Abercorn, Earl Granville, Lord Wakehurst, Lord Erskine, and Lord Grey of Naunton — occupied the house until the office of Governor was abolished when direct rule was introduced in 1972, after which the house has been retained for use on state occasions.

In 1987 the Northern Ireland Minister Tom King established a committee to advise on the structure, decoration, furnishing, maintenance and grounds of the castle. The committee concluded that the house should reflect the appearance and atmosphere of an Irish country mansion while being decorated in a manner befitting its ceremonial purposes. John O'Connell of Dublin was appointed as architect and interior design consultant, and the refurbishment was completed in 1993.