Dublin Corporation is spending more than £4 million restoring one of the finest buildings in its possession, City Hall. The work, which began in mid-November 1998, was expected to be completed this week in time for St Patrick's Day. However, the architect in charge of the project, Ronan Boylan, says the completion date has now been moved to May 26th. Sited on the corner of Dame Street and Cork Hill, and adjacent to Dublin Castle, City Hall is a superb 18th-century building which has suffered neglect over recent decades for a number of reasons. Its location on one of the capital's busiest thoroughfares has certainly not helped ensure good maintenance and the restoration now taking place is therefore very necessary. It was originally built as the Royal Exchange by the merchants of Dublin who held an architectural competition in 1768 to secure the best design. The building was to be located in a key position at the end of a vista running from Capel Street, across the river bridge and up the length of Parliament Street. Its design therefore had to be of sufficient grandeur to make an impact on the beholder even from a considerable distance. First prize was awarded to Thomas Cooley, with second place going to James Gandon, who was later to leave such a substantial mark on the city. Cooley, like Gandon, was an English architect who settled in Ireland after winning this competition; although he received a number of other major commissions, this is his finest extant work. Construction began in 1769 and ended a decade later.

The building was used as an exchange until the middle of the 19th century when a decline in the volume of Dublin's trade meant its original purpose had become somewhat redundant. It was then sold to the local authority and converted into City Hall. For this purpose a number of major alterations were made to the internal design which fundamentally damaged Cooley's original concept. A fire in the 1920s destroyed the original dome and also parts of the west side of the structure. Now, however, Dublin Corporation is taking out most of these later additions and, as far as is possible, restoring City Hall to how it looked when first built. The building was one of the very first examples of neo-classicism seen in Ireland which, until then, had been in the thrall of the more elaborately decorative rococo style.

Cooley's work has a cool character exemplified by the extensive use of pale Portland stone for both the interior and exterior of City Hall. The greatest of the internal spaces is the main rotunda and this best embodies Cooley's approach to his task. Occupying the centre of the building, it rises two storeys before meeting the great drum and dome above. Twelve immense Corinthian columns support the drum, while on each side of the rotunda are broad ambulatories. Regrettably, as part of the 19th-century alterations, these spaces were closed off from the centre and divided into offices. One of the largest jobs undertaken over the past 12 months has been the removal of all such accretions, a very substantial task as it included the extraction of chimney breasts from the ground floor while leaving them on the upper levels. When the scaffolding which now fills the entire building is eventually removed, the great hall will be filled with pale clean light, thanks to the windows which have also been restored to external walls as well as the openings around the drum. The wooden floors laid down as part of the 1850s conversion have been removed and a new Portland stone floor laid. Almost the only colour in the entire space is a deep red and some gilding used in the coffered dome. According to architect Paul Arnold, who is working on the project as a historic buildings consultant, the intention is to paint surfaces not covered in Portland stone in colours similar to this material. On either side of the hall's main entrance rise exquisite cantilevered stone staircases at the top of which, beneath glass domes, elaborate plasterwork is being cleaned and restored. The first floor rooms, most of which were created by Cooley (except for sections at the very back of the building added later in the 18th century in response to merchants' needs) are being left almost unchanged. But the other area which has been totally overhauled is the basement, or lower ground floor as those involved in this venture prefer to call it. City Hall's site slopes steeply from east to west, with the latter approaching the course of the Poddle river flowing below. As a result, the building has always been at risk from subsidence and damp. These threats have been tackled and, as far as possible, removed, allowing the lowest level of City Hall to be given a new purpose. As with the floor above, old internal divisions have been removed to create a sequence of spaces which will house a permanent exhibition telling the history of Dublin and its government. Many of the city's archives which have never been on display before - medieval charters, rolls of honorary freemen, Dublin merchant guild rolls - are to go on show, along with such items as the great Dublin civic sword dating from the end of the 14th century, the 13th-century civic seal and the great mace of 1665.

Granite floors (with heating beneath them) have been laid and display cases are being made to fit into the apses of the central exhibition room.

Evidence of the work taking place can already be seen in the sections of City Hall's exterior which have been cleaned and from which scaffolding has at last been removed. When the project is completed, in two months' time, there can be no doubt that this building will be rediscovered as an architectural gem overlooked for too long. And the reputation of Thomas Cooley, overshadowed by that of his more successful rival James Gandon, ought to receive a much-deserved boost from this restoration.