The first building on Grand Canal Square - beside designs by 'starchitects' Daniel Libeskind and Manuel Aires Mateus - more than holds its own, writes Emma Cullinan

In the context of the whole of Grand Canal Square in Dublin's Docklands, this was a bit of a Cinderella brief. At one end is the proposed theatre by internationally famous Daniel Libeskind, to the east side is a hotel by Portuguese architect Manuel Aires Mateus and in the centre there is a landscape by world-renowned Martha Schwartz.

And on the remaining side is a speculative office, restaurant and shop development by Irish practice Duffy Mitchell O'Donoghue. "We didn't want our office to be the poor relation," says partner Coli O'Donoghue standing beside the distinctive tinted glass building that, amid years of discussion about the other edifices on the square, stands alone as the first building to be completed here.

The 13,935sq m (150,000sq ft) structure went up relatively quickly along the fast-track route of the DDDA's Section 25 planning, while the other projects, announced in 2004, are still under construction.

Architects de Blacam and Meagher were to build the hotel but were surprised, having designed it, to find they had been replaced by the Portuguese architect. Now the original developer of the hotel and theatre, Devey Group, has changed. Meanwhile, Dublin architects McCauley Daye O'Connell are working with Libeskind and Mateus on their buildings.

Yet for all of the drama surrounding the other structures on the square, the DMOD office development seems to have sailed quietly into place, and that sums up its design; at first glance it offers a smooth, serene façade but stay awhile and you will see it dance. That is down to the glazing (by Schott) that comprises a dichroic plane squeezed between two panes of glass which plays with daylight and takes colours from it much in the way a rainbow does. O'Donoghue liked the way American artist James Carpenter used it in his sculptures and installations.

The building changes colour before you: from blue panels up top and projecting yellow fins, then, within seconds, to purple and green. These fins are low-iron making them look whiter, and less green when viewed side on.

Much research went into the glazing (which has a U-value of 1.2) and the Austrian company GIG Fassadenbau did a full-scale mock-up of part of the façade. "We had a great client in Roy Strudwick. He really wants to try things," says O'Donoghue. "But he wasn't sure about the glass until we went and saw the mock-up in Switzerland - comprising two panes and a fin - and he absolutely got it."

It has been a clever choice because, while the glass offers a multi-coloured dream coat, it also becomes a backdrop, providing a foil to Libeskind's theatre and reflecting the soon-to-be-present granite panels on the hotel opposite.

"I love the whole nature of glass," says O'Donoghue, describing the way it is both a liquid and fragile and sharp, and can appear both solid and transparent at different times of the day. As well as creating colour it also sometimes reflects what is around it - mainly clouds and cranes at the moment.

At night the building is designed to continue to animate the square by glowing - in different colours if so required (LED lights within can emit any number of shades). The changing hues also offer an engagement with the building without physically interacting with it.

Working with the other structures on the square was important. Apart from giving its research on non-reflective glass to the interested Mateus, the DMOD team took a weekend trip to Libeskind's office in New York to discuss the public space between them. Before deciding on Schwartz's "red carpet" the buildings didn't really connect, says O'Donoghue, "and the square was so sloped that you couldn't see the water from the front of the theatre". What they will all do, happily, is conceal the dull buildings to their rear.

The relationship with the canal basin has been important in both elevation and plan: from the rippling of light across the glass mimicking water movement, to the huge cut-outs in the east façade - overlooking the basin - and the north face, looking onto the square. Here workers can sit out - either people or water watching - in a space that resembles the deck of a ship.

The reinforced concrete frame building had to comply to the DDDA's requirement to have the top floor set back and the architects opted to curve the corner up here - mimicking the curved glass betwixt sqaure and water side at its base. The wide balcony beside the glass on the top floor mimics the deck of a ship.

Above this are overhanging glass flaps that create a nice hat on the building giving it a considered finish as opposed to just letting the façade end where the glass stops.

Looking beyond the kinetic glazing there is more evidence of careful design in the elegant proportions and considered elements: where the columns rise up through the building the glass breaks into non-reflective towers and the columns hit the ground without the glass cover, bringing the square underneath the façade and into the building. What you see is what you get here: "It is a very honest building. You can see how it is put together and its materiality. It's not like buildings that appear to be made of stone and yet the material is just millimetres thick and stuck on," says O'Donoghue.

The neatly spaced fins animate the façade but they also have a structural role in stopping the wall from twisting. "There is no excessive fat on this building, everything is doing a job," says O'Donoghue. They add the lightness of touch sought, externally by the designers. The floors are angled away from the skin: "I wanted to get rid of bloody spandrels," says O'Donoghue.

Inside, however, things get heavier with cave-like dark granite stairwells and reception area offering a negative to the lightness of the external glass, as well as to the granite squares that will cover the hotel opposite.

But there is also more play on light with the tall glass elevators allowing views up and across from reception into the atrium. These will have coloured lights shining onto their bases that will gradually fade as the lifts rise. Workers who seek animation and inspiration during the day can watch these beam-me-up comings and goings, as well as regarding people passing across the glass bridges spanning the atrium and those taking tea on the vast balcony. Above the atrium is a white airplane-like ceiling letting light in through glass on one "wing" and reflecting light into the building across the solid white other "wing".

Grand Canal Square is certainly going to be some public space when it is complete, probably next year. Ireland will have its first sculptural starchitect building - the type of wicked wobbly structure that countries often only let outsiders do, while keeping home-ground designers on the straight and narrow. It would be truly strange if this first building turned out to be the best of the lot.