Imagine a new version of Sleeping Beauty in which the princess, when she eventually stirs from her long slumber, has retained almost no trace of her good looks and certainly no awareness of her former rank.

This, in effect, is the story of Dublin after the 1801 Act of Union, when the city lost its status as a capital and fell into a provincial desuetude from which she has still not recovered. The impact of a 200-year-old piece of legislation remains insufficiently appreciated, even while its consequences continue to be felt.

For Dublin, and as a result for the entire country, the Act of Union was an enormous disaster because the fundamental character of the city altered, and not for the better.

To recognise fully the Act's importance, the development of Dublin during the 18th century must be understood. In less than 100 years, the city acquired a wondrous new appearance thanks in part to economic and political stability, but also because Dublin had become the acknowledged capital of Ireland.

The great civic structures erected during this period - the Custom House, the Four Courts, Trinity College, City Hall, the Rotunda Hospital - still define the spirit of Dublin better than anything subsequently built in the city.

And the form of the capital's centre remains largely that defined during this period, thanks mostly to the work of the Wide Streets Commission, established in 1757. As a result of this body, Ireland's capital came to possess the wide thoroughfares and clear lines of articulation only introduced to London by John Nash at the start of the following century and to Paris by Baron Haussmann from the 1860s onwards.

At the heart of the city, both literally and metaphorically, stood the Houses of Parliament designed by Edward Lovett Pearce and extended by James Gandon. In addition to acting as the legislative body for the country, this building served two other purposes: its grandeur articulated the confidence of Dublin as a nation's capital; and its physical presence drew to the city all who wished to play a role in defining Ireland's future.

This is why so many great private mansions - Leinster, Powerscourt, Tyrone, Belvedere, Aldborough and Charlemont Houses among them - as well as equally splendid streets, squares and crescents were constructed at this time. In his Dublin 1660- 1860, Maurice Craig estimates that during the last years of the 18th century, the city had a resident population of some 100 peers, plus of course an even larger number of MPs who sat in the House of Commons. Because Dublin was the centre of political power, they could not afford to live anywhere else.

But that changed on December 31st, 1800, when the Irish parliament ceased to exist and all power was transferred to London. Divested of its authority as a centre of legislation, Dublin lapsed into provincialism with extraordinary speed. By 1812 Maria Edgeworth could write her novel The Absentee in which the protagonist's parents, Lord and Lady Clonbrony, have abandoned Dublin to settle in more fashionable London.

The fictional Clonbronys' behaviour was matched by that of their real-life counterparts, as can be seen by the fate of so many great houses in the former capital. The Duke of Leinster sold his family's home to the Dublin Society in 1815; two years earlier Alborough House had become a school; by the 1840s Belvedere House had been converted to the same use, while Tyrone House became the Board of National Education's headquarters.

Very few wealthy or influential people wished to live in Dublin; in 1848 Lord Cloncurry wrote that Mornington House on Upper Merrion Street, for which his father had paid £8,000 in 1791, was now worth barely £500.

The lure of London during the 19th century was almost irresistible.

Politicians wanting to influence events in Ireland were obliged to do so in the capital of another country rather than at home; as any reading of political literature from the time shows, Westminster held an enormous fascination for even the most nationalistic MPs.

Dublin became the centre of attention in Ireland for a mere six weeks annually when the Lord Lieutenant and his staff moved into the castle for "the season" running from early February until St Patrick's Day. Otherwise there were few or no reasons for the country's affluent and influential citizens to stay in the old capital.

As a result, trade gradually decreased; tellingly, in the middle of the 19th century, Dublin's merchants could no longer support the exchange they had built 80 years earlier and sold it to the local corporation to become City Hall.

The Wide Streets Commission, dependent on London for funding after the Act of Union, was eventually abolished in 1840. New development scarcely occurred in Dublin, with Belfast becoming Ireland's boom city of the industrial revolution.

Any major buildings erected in the 19th century - the railway stations, the breweries and distilleries, the banks and insurance company offices - were invariably private initiatives and not the result of state or civic intervention.

By the beginning of the 20th century, approximately 30 per cent of Dublin's population lived in tenement slums, often carved out of the shells of once-great 18th-century mansions. The city had the worst housing conditions in Europe, equalled only by Naples although, as was noted at the time, at least the latter enjoyed sunshine.

Deprived of its role as a capital, Dublin lost such a sense of itself and settled instead for survival. And this remains true of the city even today, after almost 80 years as the capital of an independent state. The buildings that define Dublin continue to be those put up in the 18th century just as the layout of the centre is that created by the Wide Streets commissioners.

The most celebrated features of Dublin today - the city's principal squares and thoroughfares, as well as its civic structures - are invariably 200 years old and owe their inception, if not execution, to a previous incarnation. It is difficult to think of a single building erected in Dublin since independence which is of equal stature to those built in the 18th century.

The city has no 20th-century buildings anywhere near as much admired as the Custom House or the Four Courts. When national institutions require new headquarters, old stock is normally utilised for this purpose: in the middle of the last decade, for example, Government Buildings was relocated into a piece of Edwardian froth on Upper Merrion Street.

Opportunities to re-articulate the city's role as a capital by the creation of a new generation of great buildings have been ignored. Indeed post-independence Dublin has hardly been enriched even with architecture worthy of a capital. Instead, the city has seen much of its fine old building stock replaced by mediocre, if not provincial, work.

This is the unhappy long-term legacy of the Act of Union for Dublin: a city without the self-confidence to behave like a capital and prone to depend on a long distant past rather than acknowledge present realities.