Heneghan Peng's Aras Chill Dara, which has sloping glass walls, is architecture with a capital A says Frank McDonald, Environment Editor, writing about the building which opened to the public yesterday

Once occupied by the Royal Dublin Fusiliers and, more recently, the Army Apprentice School, little now remains of Devoy Barracks in Naas other than a decaying clock tower building from 1813. And its setting has been totally transformed since the barracks was closed by the Department of Defence in 1998.

It gave eight acres of the 22-acre site to Naas Town Council and sold the rest to the council for €9.35 million on condition that it was rezoned for housing. The council sold on the eight acres it got free to Kildare County Council for €2.75 million, and the balance is to be developed for social and affordable housing.

The county council had been housed for years in St Mary's, the former fever hospital in Naas, and was anxious to get out. In any case, there were plans to develop St Mary's as a "health campus", so the space it occupied was needed.

The council also wanted its own purpose-built civic offices, after years of making do.

Instead of just commissioning anyone to design the project, it did the right thing in 2000 by promoting an international architectural competition.

Roisín Heneghan and her partner Shih-fu Peng, who were working in New York at the time, emerged as the winners - which was as much of a surprise to them as everyone else.

But they had paid close attention to the council's brief, which specified that the building "must express that it is the home of the democratically elected representatives of the people [ and] provide a secure working environment . . . while being organised in a clear and simple manner for the public who avail of its services".

Drawn up by county architect Brian Swan, the brief noted that the new civic offices being built by local authorities around Ireland were also "major public statements in a country stepping out from the shadow of a bureaucracy and administration inherited from a colonial past, to being embodiments of a new self-reliant future".

Heneghan and Peng conceived it as a project "formed around the public space of the people", with a slowly inclined ground plane that gradually ascends from the street to create a "civic amphitheatre" for Naas. This was intended to expand the public realm by exploiting the depth of the site, opening it up to the town.

The two blocks that form the civic offices are seen as a continuation of this amphitheatre.

"Their inclined façades form a seamless continuity with the grass surface so that building and park no longer read as two distinct elements but rather combine to create an outdoor room - the space of the people", as the architects put it.

They saw this as giving Aras Chill Dara its "source of identity". But in reality, it is defined by architecture with a capital A. Seen from the Newbridge Road, the first impression is startling - a pair of glass blocks with sloping walls and screens linked by a transparent four-storey bridge enclosing suspended ramps.

Heneghan and Peng justify the tilted form and splayed corners of the office blocks by saying it is all about folding the architecture to embrace the garden, turning it into a "natural room". And though they say "sheer cliffs" would have been inappropriate, someone cheekily offered builder Ged Pierse a plumb-line for Christmas.

The pedestrian route leading to the main entrance is flanked on one side by an almost Egyptian stone wall, inclined at the same angle (6 degrees) as the glazed screens of the office buildings, with their baked ceramic pattern of a multitude of green droplets; the effect is almost like walking through a tall, angular pergola.

A retained mature oak tree gives some gravitas to the garden, which is being landscaped by Mitchell and Associates. It includes reflecting pools, a public sculpture, and tiered stone seating parallel to the front of the clock tower building; its arch is envisaged as "a gracious entrance for performers onto the stage".

But it would be impossible for anyone other than a child to stand up on the upper stone tiers because they are oversailed by the county council's chamber, which is cantilevered out 30 ft (nine metres) - too close to the old building. Its dark undercroft, over the pedestrian route from the car park, is also quite oppressive.

By contrast, the expansive entrance foyer is bright and welcoming. At the far end, beside Naas Town Council's offices (defined by their curving form) is a giant-sized equestrian sculpture entitled Óisín in a Timewarp by the late James McKenna. Donated by poet Des Egan, it it seen as fitting for a "thoroughbred county".

The whole building is organised around the ramps linking its two parallel blocks, which the architects call "bars".

Because one block is half a level lower than the other, the ramps have a gentle slope interspersed by flat landings and are suspended in space by centrally positioned steel columns made by Eiffel in France.

The maple ramps not only make the building fully accessible, but easy for visitors to "navigate" the new offices and find their way to the well-planned public counters for motor tax, planning and housing. They were also designed to encourage impromptu encounters between different departments as well as the people they serve.

Environmental sustainability was also very much part of the agenda. The office blocks are just 12 metres in width to allow for natural ventilation and optimum daylight. Reflector strips on the smooth concrete ceilings even bounce daylight into the middle of each floor, and artificial lighting is also attuned to daylight levels.

Solar panels on the roof pre-heat water for the wash-hand basins while rainwater is recycled for flushing toilets, all of which are disabled-accessible. As in ABK's civic offices in Nenagh, Co Tipperary, porous paving in the parking area allows rainwater to seep away naturally, reducing surges in the storm drainage network.

The project, with an all-in cost of €58.5 million, was designed in association with Arthur Gibney and Partners along with a team that included Buro Happold, Michael Punch and Partners and RFR. Superbly built by Pierse Contracting, Ged Pierse has hailed it as their best building yet - even though it was hardly the easiest.

"It will be recognised as one of the best buildings in Europe in the first decade of the 21st century", he said. Compare and contrast its challenging contemporary architecture with the woeful new shopping centre on the Dublin Road entry into Naas; even as a piece of pastiche, this would surely be rated as the worst of its type in Ireland.

Aras Chill Dara is also the first major building to be completed by Heneghan Peng Architects and takes their work from the theoretical to the tactile. What's yet to come includes two more competition-winners - the Grand Egyptian Museum near the Pyramids of Giza and a more modest visitor centre for the Giants Causeway in Antrim.