Remember the old Cats and Dogs Home on Grand Canal Quay? Look for it today and what you'll find on the site is a seven-storey office building with a shimmering glazed facade that acts as a modern mirror for the 19th century canal-side malthouses directly opposite.

Designed by de Blacam & Meagher, the new Esat headquarters deservedly featured among the winners in this year's Architectural Association of Ireland's awards, presented earlier this month.

Denis O'Brien, the founder of Esat, obviously wanted to make a major corporate statement when the company's headquarters was being planned in 1997. By the time the building was finished, however, he had sold out to British Telecom - though he still retains the penthouse, presumably as a top-drawer tenant.

At 81,270 sq ft, this is a large building with seven floors of office space and two levels of car-parking in the basement. But the architects have managed to slot it quite gracefully into its Grand Canal Quay context by stepping back the fourth and fifth floors above the parapet level as well as the penthouse.

This is an area in transition and it cannot be long before other new buildings of significant scale replace the remaining gable-fronted sheds between the Esat headquarters and the tall limestone tower of Trinity's (formerly the IDA's) Enterprise Centre on Pearse Street. Even toytown Clanwilliam Square must surely be in the melting pot.

Certainly, Esat had no problem on the planning front, winning an uncontested permission in just six weeks for what Dublin Corporation's planners could readily see as a building of real quality. The manner in which its height was handled did help, of course, but there was also the precedent of the Treasury Building on Grand Canal Street.

Esat's triangular site, hard by the edge of the railway tracks spewing out of Pearse Station, was exceedingly difficult to develop. The bridge carrying the railway has only a clearance of eight feet, so the only access to the site was from the Pearse Street end - and many of the trucks had to reverse in because there was nowhere to turn. Adrian Buckley, de Blacam & Meagher's project architect, is particularly pleased with the new bridge that links the Esat building to the malthouses across the street. A light and elegant steel truss with a glazed balustrade, it contrasts with the mass of the railway bridge - even if the Esat corporate fallout means it is now rarely used.

Raising the building on a plinth of riven slate was a reference to the solid ground floors of older buildings in the area; its texture also counterpoints the smoothness of the glazed facade. But there were practical reasons for bringing the ground floor up half a level - to accommodate an ESB sub-station and get two levels of car-parking. The Pilkington planar glass wall, made in Britain by Portal, faces east and comes with canopies to provide shading from the morning sun. Sensors shoot the blinds out automatically (and retract them in high winds), but individual employees have the option of over-riding the system simply by pressing buttons on the sides of their desks.

Originally, the building was designed to be naturally ventilated. But as in so many other cases, the architects lost the argument with the chartered surveyors - Jones Lang LaSalle - over whether this was really a runner in terms of protecting capital values. So it has ended up being air-conditioned and, therefore, hermetically sealed.

This is tragic. Whatever about the impact on the glazed facade of having windows that actually open, it is beyond time - as the Fingal County Hall has shown - for the gurus who advise developers to wake up to the fact that there are more environmentally-conscious ways of designing buildings. Or is it that they are incorrigible?

In summertime, huge amounts of energy must now be consumed to cool the building and its six-storey, south-facing atrium which would otherwise - had the architects got their way - have acted as a ventilation `stack'. Suspended canvas screens and bamboo provide shade here and would probably have been needed in any case.

The slate plinth is returned inside to flank a flight of Portuguese limestone steps leading up to the raised ground floor, giving the building a grander entrance than most office blocks. The uprights on the stairs are stainless steel and the rails are oak, as is the security desk, which has a rectangular pool cantilevered over it.

Ingeniously, disabled access is via a downward ramp behind the security desk to the lift lobby on a lower level, from where anyone in a wheelchair can travel to any part of the building. But what's really stunning is the ultra-smooth, almost polished finish on the concrete columns, cast in shutters lined with PVC or fibreglass. Beautiful!

The atrium, however, seems somewhat off-centre - and it's a pity that the lounge furniture here is catalogue stuff rather than specially commissioned. The dark brown powder-coated aluminium framing of the glass also appears rather chunky - though it works very well in a slimmer version on the flush windows at the rear.

T-PLAN in shape, the Esat headquarters extends quite a distance to the rear, terminating almost in a point. This is where one of its three staircases are located and, unlike the alienating closed boxes in most office buildings, there are great views outwards from every landing. "We tried to make them into interesting places," says the project architect, Mr Buckley.

The brick cladding is an attractive mix of standard stock and overburns, imported from Britain, though there is some efflorescence on the north wall. Inside, meeting rooms and cellular offices are ranged on this side of the building, so that staff working in the largely open-plan office spaces enjoy the maximum amount of natural light.

The concrete-framed building is unusual in that its exposed floor beams, which incorporate recessed downlighting, taper upwards to a very narrow edge where they are connected to the walls by mild steel plates known as "wind posts". On the front, these posts are tied to stainless steel "spider" fittings to hold the glass wall in place.

There are planted roof terraces on the fourth floor, where Esat's boardroom is located, and at the penthouse level, which has a full-length steel pergola with timber laths. The ground floor also has its own garden, with raised beds bordered by railway sleepers (what else?), flanked by a linear ventilation shaft for the car-park underneath. The gardens are all tended, as they should be, and in time Denis O'Brien will have clematis crawling all over the pergola outside his "Master of the Universe" suite. But there has inevitably been some snags - the fourth-floor terrace is leaking, one of the staircases isn't tiled and there is an ugly wall of breeze-block just inside the carpark.

And though the Esat headquarters is next door to the new Grand Canal Docks DART station, those among the 600 staff who travel by train must walk all the way around the block for half-a-mile or more to get to it, because Iarnrod Eireann failed to provide a second entrance. "It's silly, and so absolutely Irish", Adrian Buckley fumes.

On Tuesday, April 10th, a report in The Irish Times on the AAI Awards incorrectly stated that the Irish Pavilion at last year's Hanover Expo was to be brought back for re-erection in the Cherrywood Science and Technology Park. In fact, it is to be installed at the Dun Laoghaire Institute of Art, Design and Technology.