I keep hearing about the plant rooms. The director of the National Gallery of Ireland, Sean Rainbird, is particularly keen for me to view these concrete vaults, never to be seen by the general public, containing the ducts and machinery that will keep the galleries at the right temperature and humidity to preserve the treasures on show.

There’s a reason for his enthusiasm: the gallery is the only cultural institution to have achieved a major capital project since recession hit the country, and about a third of the €30m construction budget of its just finished renewal has gone on the mechanics, electricals and fire precautions that enable the gallery to function.

There are other intangibles. The gallery’s existing buildings are a series of accretions and adaptations since its first block opened in 1864 that left it with a parade of magnificent, elegant and sometimes mismatched spaces but also congested and confusing, so it has been given the architectural equivalent of a Vicks nasal spray. Blockages have been vaporised. Generous windows with delicate frames, that had been bricked up, have been uncovered, which reduces hanging space but increases the lucidity and delight of the galleries. The previously cluttered area in front of the building has been remade, such that it converses better with other venerable institutions around it, with a broad sweep rising to the entrance and a lawn covering the famous plant rooms beneath.

Facebook Twitter Pinterest Uncluttered: a broad sweep now rises to the entrance of the gallery. Photograph: Marie Louise Halpenny

Effort has gone into ensuring that the daylight that falls through the 19th- and early 20th-century rooflights feels like daylight. Often in art galleries the need to protect paintings from ultraviolet light means that whatever comes from the sky is filtered and managed to the point that it might as well be artificial. Here it is allowed to vary with the seasons – more in summer, less in winter – such that, while the annual total of ultraviolet is the same as other galleries, it can periodically become brighter and livelier.

Facebook Twitter Pinterest Photograph: Marie Louise Halpenny

The architects for the renewal, the Irish and American Roisin Heneghan and Shih-Fu Peng, are not obvious casting for a work of subtlety and nuance. Their buildings, such as their library and academic building for the University of Greenwich, tend to have a certain meatiness. They like to be emphatic and expressive and to operate in units of decimetres rather than millimetres. They hadn’t worked with historic buildings at this scale before.

In one space you can see their signature, a central glazed-over courtyard formed out of an old lightwell. The 19th-century fabric of bricks, glazed bricks and stone, grey and off-white, is augmented by concrete lift-towers, the marks of their casting left visible. It’s hard-surfaced and reverberant but it comes to life when sunlight washes round it. It does its job, which is to relieve and refresh the crowd of rooms around it and provide a fixed reference point in the potentially disorienting layout of the older buildings.

Facebook Twitter Pinterest Magnus Modus by Joseph Walsh in the new gallery courtyard. Photograph: Marie Louise Halpenny

In places, Heneghan Peng’s touch lets them down. The stairs that rise next to one of the lift towers could do with a bit more joy and delicacy. The courtyard could be a touch more hospitable – a random looping sculpture, Magnus Modus by Joseph Walsh, seemingly installed to liven things up, doesn’t really help. But the new works do what they should, which is to make sense of the older buildings and to help them breathe.

According to Rainbird, the gallery would have had to close if the works hadn’t been done, for failing to meet modern fire regulations. This would have been a cultural catastrophe. With its works by Goya, Caravaggio, Rembrandt, Monet and Vermeer, and by Irish artists such as the older and younger Yeatses, built up with modest budgets and generous donations, the gallery is a miraculous achievement for a small and often cash-strapped country. For this reason, government ministers worked to find chinks in the severe restrictions of the “troika” – the International Monetary Fund, the European Central Bank and the European commission – on the nation’s budget, to pay for the gallery’s renewal.

Now Rainbird and Heneghan Peng want to build another phase, with gallery and education space, which will sort out still nonsensical aspects of its circulation. For this they will need more cash from the recovering but still hard-pressed Irish exchequer, which will require more of the resourcefulness and courage shown in getting this far. But the gallery is worth it.

The National Gallery of Ireland reopens on 15 June