"The most remarkable thing now going on is a house of Mr Conolly's at Castletown, it is 142 feet in front and above 60 in the clear, the height will be about 70. It is to be of fine wrought stone, harder and better coloured than the Portland, with outhouses joining to it by colonnades, etc. The plan is chiefly of Mr Conolly's invention, however, in some points they have been pleased to consult me." Bishop Berkeley 1722
Castletown, as Ireland's first and largest Palladian style house is an important part of Ireland's architectural heritage. Erected between 1722 and c.1729, Castletown was built with two wings connected by Ionic colonnades flanking the Renaissance inspired central block of the house. The wings in true Palladian fashion contained the kitchens on one side and the stables on the other. This style had originated in Italy with the 16th century architect Andrea Palladio (1508-80), and had come to prominence in England in the early eighteenth century. The original interior layout of the house owed much to recently published plans of English houses such as Chevening in Kent, with a central hall and saloon surrounded by four apartments on the ground floor and a gallery flanked by apartments on the piano nobile level. This primacy of the first floor was emphasised by longer widows on the façade (the lengthening of the ground floor windows later in the century distorted this effect). The layout reflected Conolly's vision of the house as a venue for large scale political entertaining.
The identity of the architect of the house is still subject to debate but the façade, built of Ardbraccan limestone is by the Italian architect Alessandro Galilei (1691-1737), whom Conolly had met when the former visited Ireland in 1718-19. When construction began three years later Galilei had returned to Italy, and it is unclear whether his plans were followed. Instead it seems likely that Conolly sought advice from a number of local connoisseurs including the philosopher, George Berkeley, who had recently returned from an extensive Italian grand tour, as well as the architect Thomas Burgh. The initial building work seems to have been overseen by Irish master builder John Rothery, later architect of Mount Ievers Co. Clare, which shares a number of features with Castletown including the two gigantic chimney stacks, the plain unimbellished facade and the corner fireplaces. Edward Lovett Pearce, the young Irish architect, upon his return from his Grand Tour in Italy, where he had met Galilei in Florence, in 1724-25, added the service wings and the colonnades. He also seems to have been responsible for the interior layout of the house, and a ground floor plan survives amongst his papers. Pearce (1699-1733) who also designed the Irish House of Commons in College Green (a commission he secured partly through Conolly's patronage) was the leading Irish architect of the early eighteenth century. His addition of the Ionic colonnades and the Palladian style wings was to influence the design of many of the great eighteenth century Irish houses such as Carton, Co. Kildare and Russborough, Co. Wicklow both designed by his assistant and successor Richard Castle (1690-1751).
Castletown underwent a radical architectural transformation following the arrival of Lady Louisa Conolly in 1759. Over the next forty years she spent over £25,000 on improvements to the house and demesne. Guided by her brother-in-law the Duke of Leinster and the published designs of leading British architects Sir William Chambers (1723-96) and Isaac Ware, Louisa altered the layout of the interior, remodelling the main reception rooms including the dining room, the two drawing rooms and the magnificent long gallery, as well as the great staircase built in 1759. She also altered the front facade of the house, lengthening the windows to fit in with contemporary fashion, giving the ground floor equal emphasis. These changes reflected the changing function of the house as the Conollys made it their permanent residence. A constant stream of informal visits replaced the political congresses intended by Speaker Conolly, and later hosted by his wife. Following Lady Louisa's death in 1821 few substantive architectural changes were made to the house although ambitious plans were drawn up in the 1850s, by Tom Conolly (1823-76), to cover in the stable yard behind the east wing, but never carried out. The main reception rooms were, however, extensively redecorated, probably in the 1850s. This work included the conversion of the state bedroom into a library and the print room into a billiard room, as well as the replacement of the silk in the drawing rooms. The long gallery was also extensively renovated, while a Gothicised smoking room was also provided above the stables.