Santiago Calatrava, it might be imagined from the spectacular structures he designs, should be a flamboyant, larger-than-life figure, like Falstaff or the late James Stirling. In fact, he is the reverse - diminutive, softspoken, even slightly self-effacing.

Calatrava, the world's preeminent bridge-builder at the age of 50, flew in to Dublin last week (on his way from Paris to Brussels) to explain his design for a dramatic, 120-metre cable-stayed bridge in the Docklands area at an An Bord Pleanala oral hearing in the Gresham Hotel.

With his thick black hair, bushy eyebrows, sharp suit, gold watch and designer specs, he certainly knows how to "talk the talk", even if it comes flowing in a still-heavy, but charming Spanish accent, peppered with words and phrases that show his first language was Catalan.

One of the keys to Calatrava's success is that he trained first as an architect and then as an engineer, so he really understands structure. And the emblematic quality of his work has led to commissions to design bridges in Spain, France, Germany, Belgium, Italy, Poland, Greece, Switzerland, Argentina and the US.

In January 1999, Dublin Corporation appointed him to design two new bridges across the River Liffey - one linking Blackhall Place with Ushers Island and the other in Docklands, mid-way between the Matt Talbot and East Link bridges. Design and visual impact, rather than mere functionality, were the principal criteria.

Last July, in the Observer, critic Dejan Sudjic suggested that the commissioning of "signature" bridges was "the unmistakable sign of a city with an inferiority complex struggling to make its mark". Why turn to an international superstar, the argument goes, when we should have enough home-grown talent to design bridges?

And it must be said that the Valencia-born architect-engineer and his show-off structures are not popular among architects. Reflecting their rather jaundiced view, one Dublin architect paraphrased Lady Bracknell: "To have one Calatrava bridge may be regarded as a misfortune; to have two looks like carelessness."

But Tim Brick, the deputy city engineer, said it was "nonsense" to suggest that Dublin Corporation was "branding" the city. "What fascinated us was the combination of pure mathematics - the fireworks, if you like - and the architectural treatment. Gone are the days when Ireland should be constraining itself to simplistic design."

And the truth is that Calatrava has captured the public imagination by turning bridges into spectacles. What made his name internationally was the stupendous bridge he threw across the Guadalquivir in Seville - one of 11 built for Expo '92 - cable-stayed and suspended from a leaning pylon that symbolised its structural tension.

The sculptural quality of Calatrava's bridges, some of which swoop like an extended long-bow, derives from his admiration for organic structures and for Gaudi's work in Barcelona - even if, as Sudjic wrote, he has "added a dash of Dan Dare and a pinch of art deco to his enthusiasm for genetically-modified Gothic architecture".

At last week's Bord Pleanala hearing, the maestro talked about how bridges "have the power to generate cities" and said this had been true since Roman times. Even more than museums, bridges have millions of users so they are "fundamental to the quality of life" - something not understood by the highway engineers of modern times.

"We have built more than 50 bridges over the past 20 years so we know something about it," he declared, with uncharacteristic immodesty. And what he and his very busy Zurich-based practice produces represents a significant departure from the conventional highway engineering approach to bridge design.

He cited his new bridge over the Loire, outside Orleans, where the parabolic arches were made as thin as possible so that this "artificial object" would fit into its riverine parkland setting. And though designed to carry traffic, the bridge has very wide pedestrian walkways which double as "safe spaces for children to play".

As someone who doesn't drive a car and describes himself as "a real pedestrian", yet travels all over the world, Calatrava wants his bridges to be enjoyed by strollers as much as used by motorists. So he is particularly concerned about the quality of the "pedestrian experience" in trying to make his bridges as "friendly as possible".

Asked how he felt about the fears of Docklands residents of having an extra 2,000-plus vehicles clogging their streets, he accepted that there was a certain inevitability about the Macken Street bridge generating an increase in traffic, but said: "The key thing is to create something people will be proud of and adopt as their own."

Calatrava cited the example of Murcia in the south of Spain where apartments with a view of his new bridge there, stunningly illuminated after dark, had substantially increased in value since it was built. (This is not much comfort to residents of Macken Street, most of whom will see nothing but more traffic from their two-storey houses.)

In form if not in scale, the new bridge in Murcia is oddly similar to the one now finally under construction at Blackhall Place. The chief feature of both is a pair of splayed, parabolic steel arches from which the concrete deck is suspended by high-tensile hangers. And as in Murcia, the Blackhall Place bridge will be dramatically floodlit.

So whatever about working with local consulting engineers (Roughan O'Donovan, for both of his Dublin bridges), is he not just another globalised brand? His answer is that while engineering achievements, materials and technical resources are common to all developed countries, "the nature of the problem is always different".

In the Macken Street case, a particular technical solution was required so that the bridge could swing open for navigation, at least as far as the Matt Talbot bridge. That involved designing a curved pylon which would rise from a pivot firmly anchored in the riverbed. "This bridge is unique; it has never been done before," Calatrava declared.

Producing a 50p coin from his pocket, he said the design was inspired by the Irish harp on its obverse, giving it a certain singularity. It had also evolved over time, which explains why its current form differs somewhat from the original, rather sharper profile conveyed by images of the model made in 1999 when it was first unveiled.

Naturally, he rejected any suggestion that the latest version is blander, preferring to describe it as "more mature". He was also emphatic that the height of the pylon was critical to retaining the bridge as rigidly as possible, even in an open position. Otherwise, there would be a danger of the deck sagging and it might slot back into place.

As for the Dublin Docklands Development Authority's view that the bridge should be skewed to connect Macken Street directly with Guild Street on the north side, Calatrava made a convincing case that all bridges should be perpendicular to the river walls, as they are in Paris and - Matt Talbot excepted - in Dublin, too.

Having visited Dublin many times, unlike most of the superstars jetting in here in recent years, he came to understand that the Liffey is "the spine of the city" and the quays would therefore always be more important than any of the adjoining streets. He also wanted to add to the "rich patrimony of bridges" spanning the river.

There were practical considerations, too. If the Docklands bridge was built at a skewed angle to the Liffey, as the DDDA would prefer, this would add some 20 per cent to its length and 40 per cent to its materials. The pylon would also have to be thicker and there would be a need for more cables - in other words, a new design.

No more than other "signature architects", Calatrava doesn't come cheap. But his two bridges for Dublin would still cost no more than £20 million - a fraction of the figures being bandied about for the Bertie Bowl and Sports Campus Ireland. And on a sliding scale, bridges are more important to cities than sports stadiums.

"Bridges are not only links - they can become places, too," he said, "and that's why they should be objects of interest and beauty." In that context, we shouldn't forget about replacing the hideous Loop Line bridge with something more transparent - and we don't need Calatrava to do that because it's been designed already.

All we need to find is the pittance to pay for it.