A 1970s office block is the first Government building to be deep retrofitted to reduce its carbon emissions.
Tom Johnson House at Beggars Bush in Dublin will use 75% less energy and the building's life will be extended by more than 50 years.
The two years of work at the site is almost complete.
Hundreds of employees with the Department of the Environment, Climate and Communications will move in within weeks.
The retrofitted building will also house the National Cyber Security Centre.
Principal Architect with the Office of Public Works (OPW) Ciarán O Connor said: "What we've done is taken an old building and given it new life.
"This is giving the building at least another 50 to 80 years."
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Tom Johnson House, previously home to the Labour Court, was built in 1973.
It is State-owned and was selected by the OPW as the first Government workplace to undergo a deep retrofit.
"If we are renting buildings, we don’t get the value of energy saving," Mr O’Connor said.
"By doing it to our building we get the long-term gain," he added.
To avoid thousands of tonnes of carbon release, the exterior and concrete were retained.
"This building has its carbon since 1973, so we are not adding to that," he said.
If we were to knock it down and rebuild it, we would be," he added.
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The focus was on making the interior more effective and efficient.
The old building had an energy rating of C3 while the retrofitted building will be an A2.
The OPW expects energy use to fall by 75%.
Senior architect at the OPW Dan Costelloe said: "This has been achieved by heavily insulating the building, using a heat pump and photovoltaic panels on the roof."
Natural ventilation and more light will make it easier to regulate temperature.
"Dark and gloomy" is how Mr Costelloe recalls the old Tom Johnson House.
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"The building had these long corridors that stretched on forever," he said.
The new light-filled central atrium will connect the building.
The building’s' three open plan spaces can accommodate hundreds of workers comfortably.
In the coming weeks, employees from the Department of Environment, Climate and Communications will occupy the retrofitted building, bringing some furniture from their existing offices.
The carpet and paint has recycled content and the OPW said that 90% of waste material taken out has been recycled.
With just 70 car spaces, Mr Costelloe said the new design has cyclists in mind.
"We have a large internal cycle parking area, changing rooms and drying rooms," Mr Costelloe said.
All the facilities are there to encourage staff to cycle to work," he said.
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By 2030 Ireland must reduce carbon emissions by 51% and reach net zero emissions by 2050.
All buildings housing Government departments will have to become more energy efficient.
The UN said buildings account for nearly 40% of energy related emissions globally.
Up to 20 Government offices are thought suitable for a deep retrofit.
But work on Tom Johnson House has taken almost two years and the work is costly.
RTÉ understands the final cost of this "exemplar" project has yet to be determined, but will top €20 million.
The Tom Johnson House retrofit has been funded to the tune of 70% by the European Commission, with the taxpayer picking up the balance.
Further large retrofits will depend on funding and on how this exemplar building performs.
"We have a whole set of things throughout the building to measure temperature, quality of air, levels of light," Mr O’Connor said.
He said that results will be checked against design criteria to see if they matched.
"What happened in London is they found that 80% of the buildings did not meet the standard they set out to hit," he added.