After the Act of Union of 1801, Dublin entered a period of decline with the loss of its administrative power and structures. Architectural development still continued for some years with the construction of the General Post Office, St George’s Church, and the ongoing development of the Fitzwilliam Estate. With the bankruptcy of the Gardiner Estate in 1846, the north side of the city started its long slide into disrepair as many of the once fine Georgian houses became tenements and slums. In the later part of the century, fine Victorian structures including banks and public houses were constructed. Important buildings like the South City Markets on Great George’s Street and the Museum Building of Trinity College were designed and built. The regional railway companies constructed four fine termini in the city all based on different architectural styles. So despite the decline of Dublin into a regional city of the British Empire, the city still managed to acquire a wealth of important 19th century building.

With the loss of funding resultant from the dissolution of the Irish parliament in 1801, many of the fine and ambitious plans of the Wide Streets Commissioners and Landlords had to be shelved. The Royal Circus, a elliptical development planned for the top of Eccles Street by the Gardiners never materialised but ‘haunted’ maps of the city for many years. The construction of the city quays continued with the last of these: Wellington Quay completed in 1812. The Wide Streets Commisioners were finally abolished in 1841.

The romantic notions of the victorians meant that money was available to restore the ailing medieval cathedrals of the city. The Guinness family paid for St Patrick’s Cathedral to be heavily renovated, while a Dublin distiller Henry Roe paid for the repairs to Christchurch. At St Patrick’s, the buildings adjacent were bought, demolished and turned into a park to set off the cathedral to its pictorial advantage. Christchurch required much more intervention - the building was in poor repair. As well as demolition of part of the building - the long choir, adjacent buildings that were built against the cathedral were bought and demolished and the building linked across to the Synod House. The generosity of benefactors like these enabled Dublin to maintain many of its fine churchs and buildings.

Also evident in the 19th century was the inability of those running the city to see beyond the immediate - just after the Act of Union, the city fathers advocated filling in the centre of Stephen’s Green for housing, the redevelopment of the Mansion House as a street and the demolition of the Tholsel - one of Dublin’s medieval civic buildings. The Tholsel was demolished but neither of the other two ever came to pass. Similar suggestion occurred in the twentieth century - notably the suggestion that Merrion Square be filled with a new national Roman Catholic Cathedral.