Richard Cassels (1690-1751) was of German origin and also known as Richard Castle. He settled in Ireland around 1728 and worked with Edward Lovett Pearce on the Houses of Parliament before becoming the leading country house architect of his day in Ireland. He was responsible for some of the largest houses in Ireland including Russborough (1742-1755) in County Wicklow which has the longest frontage of any house in Ireland and Carton (1739-1745), County Kildare. He was also accredited with Summerhill, County Meath, an enormous house which was burnt in the Civil war of 1922 and finally demolished in the middle of this century. However it may be that the building was designed by Pearce before his death or at least outlined and that Cassels was the executant architect.

In Dublin his major works were: Tyrone House (1740-1745) for the Beresfords; Leinster House (1745-1751) which he designed for the Earls of Kildare; the Rotunda Hospital (1757-) which was based on a country house composition; and Powerscourt House which is now a shopping centre. His Dining Hall at Trinity College was demolished after two collapses of the foundations and was replaced in the 1760s by a design by Hugh Darley.