Roughan and O'Donovan are the engineers behind some of our best new bridges, writes Emma Cullinan

Like an elegant, spidery hand reaching down and helpfully lifting the deck of the bridge up off the traffic, the cable "fingers" of the Taney Bridge in Dundrum descend from a soaring pylon. The resulting lofty structure, forming a gateway to the south Dublin neighbourhood, looks almost ostentatious. Yet its form follows from strict functional requirements.

That's the beauty of certain bridges; the elements, and the whole, of the built form express the maths behind their construction (without showing off too much and reaching far beyond the sums of their parts). It's possible to see how all of the bits play their part in holding up the structure: here's the arm offering support, and here are the strings that are pulling up the deck.

It is not possible to see the support structure of many buildings, which is why bridges can be so pleasing to look at. There's a place in our hearts for the Clifton Suspension Bridge in Bristol, England by engineer Isambard Kingdom Brunel, who also built many expressive-but-functional iron structures; viaducts worldwide; and the Golden Gate Bridge, which has become a symbol for San Francisco.

Stunning bridges offer a break in journeys - a wow factor along the way - as can be seen in Dundrum and the new Boyne Bridge on the Drogheda bypass, which is lit spectacularly at night. Both of these are by the same firm of engineers, Roughan and O'Donovan. Both cable-stayed bridges, the first has a concrete deck and the second is made from hollow steel wrapped in GRP (glass reinforced plastic).

In today's fast-moving world bridges often have to be built without disrupting traffic, as was the case in Dundrum. The bridge has no feet on the ground - so that the road junction wouldn't be obstructed by columns.

The bridge was designed to carry Luas trams which don't like climbing or descending steep gradients. So, the requirement was for a deck that was high enough to clear the traffic, but low enough to prevent inordinate tram climb. The brief required a bridge with no legs, which could be built without closing the roads (except for a few hours on certain nights).

"This type of structure was the only answer," says engineer Joe O'Donovan, managing director of Roughan and O'Donovan. "The beauty of the Dundrum bridge is that the pylon is way back off the road and its construction didn't interfere with the traffic."

Once the pylon was constructed, the bridge was knitted into place, with pairs of cables - or stays - being attached bit-by-bit to sections of the bridge. This enabled the structure to be gradually built out over the road as traffic zoomed beneath.

O'Donovan has always had a passion for bridges, and has designed many smaller crossings around Ireland, but knew that if ever he got the opportunity to "do a decent crossing I would create something special".

As well as the design, the practicalities of bridge construction offer high antics. A pedestrian bridge that the company did in Kilmacanogue had sections of deck threaded onto long metal cables that spanned the road, so that the traffic could keep moving beneath this too. While a "cable-stay" structure was the answer to the Dundrum conundrum, the engineers took time to refine the design. "Once you've made the decision about the type of structure needed, a lot of thought then goes into the proportioning," says O'Donovan.

Hence the slender Y-shaped pylon, slim deck and the fact that the ends of cables appear on the underside of the bridge: an enchanting illustration of how the bridge is made.

The underside of the deck has a curve beneath as a people pleaser. "A huge number of people go underneath the bridge every day and we wanted to add interest," says O'Donovan.

His firm is involved in a remarkable number of bridges in Ireland, which has seen a dearth of iconic bridges, unlike in Spain, France, Italy and Germany. Those countries have a reputation for good design, but geography helped too, says O'Donovan. "We don't have the terrain of Spain, Italy, Germany and France, with mountains and big, steep valleys where you would be putting viaducts across and tunnels beneath. Most of the roads and railways in Ireland are built on flat land. There are few really big river crossings."

O'Donovan has seen Spanish design at close hand, both on his travels abroad, and through having worked with Spanish architect Santiago Calatrava, who has built his reputation on complex, curvaceous bridges. The engineer worked with Calatrava on the large scale James Joyce bridge over the Liffey. Such bridges work in an urban setting, to make a statement, says O'Donovan, something that Dublin city architect Jim Barrett would concur with, saying: "In such settings it's good to have a bridge that overstates itself."

Working with Calatrava is "interesting" says O'Donovan, "he's different to anyone else we've worked with. He has tremendous attention to detail. When you go for highly complex detailing and articulation you pay a lot more for it. That's a niche Calatrava has found for himself. People are prepared to pay for it because they feel that it's worth it. The shapes of his bridges are interesting, complex and very pleasing to the eye."

Roughan and O'Donovan are now working with Calatrava on the Macken Street bridge which has just gone out to tender. "The engineering on that is highly complex: we did an awful lot of work on it." The structure pivots open at the base of a single arm that reaches high into the air.

Much of Roughan and O'Donovan's work is on more low-key projects for contractors but even here they put care into the designs, such as in the new curved-profile Trim bridge, the sagging concrete footbridge in Kilmacanogue, and the new airport road bridges that lean in towards each other.

The appeal of bridges for engineers is that they form a key part of the process. "We are the senior member of the team, unlike in buildings where the architect leads the team, and rightly so."

O'Donovan often works with Grafton Architects (specifically Shelly McNamara and Philippe O'Sullivan): "I enjoy working with them, they are a brilliant firm and I admire the work they do. It can be a very productive process. We'll get some very good ideas coming from their side; we will work on them and then go back again with a change. You get this synergy."

In a society that tends to feel secure surrounded by traditional-style structures, it is amazing how these new dynamic, contemporary bridges have quickly found favour. Their acceptance is probably eased by the fact that they tend not to replace much-loved old buildings or classic bridges, and are not planted in people's back yards.

So here is an opportunity to build exciting structures. Now that more motorways are being planned, let's hope that the contracts insist on careful bridge design, using that creative synergy between engineers and architects, rather than just lumping bridges in with the rest of the road building.