Elsewhere in Dublin, George Semple built St Patrick's Hospital (1747) and Thomas Cooley the Royal Exchange (1769; now City Hall). Georgian Ireland In the later half of the 18th century, one of the most important architects in the country was the London-born James Gandon. Gandon came to Ireland in 1781 at the invitation of Lord Carlow and John Beresford, the Irish commissioner of revenue. Gandon's buildings in Dublin include the Custom House, the Four Courts, the King's Inns and the eastern extension to the Irish parliament building in College Green. By this time the Palladian style had evolved further, and the strict rules of mathematical ratio and axis dictated by Palladio had been all but abandoned. This subsequent evolution is generally referred to as Georgian architecture. It is in this style that large parts of Dublin were rebuilt, causing the city to be referred to as Georgian Dublin. Francis Johnston was the third great Irish architect of this period. Johnston served as Architect to the Board of Works for a time and was thus responsible for much of the planning of Georgian Dublin. He also left a number of very fine buildings, including St. George's Church, Hardwicke Place and the Viceregal Lodge in the Phoenix Park. This latter now serves as Áras an Uachtaráin, the official residence of the President of Ireland, and is one possible model for the White House in Washington. Leinster House also claims this distinction, and the Neoclassical Castle Coole in County Fermanagh designed by James Wyatt bears an even greater similarity. In addition to these large-scale buildings, the defining characteristic of Georgian city planning was terraces and squares of elegant family homes. In Ireland, many of these became tenements during the course of the 19th century and a significant proportion were demolished as part of various 20th-century slum clearance programmes. However, many squares and terraces survive in both Dublin and Limerick. Of particular interest are Pery Square in the latter city and Merrion Square in the former. Some smaller towns in Ireland also have Georgian architecture of interest, such as the fine Georgian squares and terraces of Mountmellick, County Laois, and Birr, County Offaly, which is a designated Irish Heritage Town. Near the end of George III's reign, one of Ireland's most famous Georgian buildings was completed. The GPO was built in 1814 and located on Dublin's main street, O'Connell Street. Designed by Francis Johnston,