LOOKING at Dublin City Gallery, The Hugh Lane from the outside, you would be forgiven for thinking that little has changed at the gallery. The elegant Georgian facade of Charlemont House is still the centrepiece of Parnell Square, just as it was when it was built in 1765.

Except, that is, for an unobtrusive glass facade peeking out from behind the righthand wing. This is the only hint, from the exterior, of the dramatic changes that have taken place at the Hugh Lane. After a long period of closure, the gallery will reopen this Friday with an impressive 13m extension designed by Gilroy McMahon. With 13 new gallery spaces and 2,000sq m of extra floor space, the gallery has doubled in size.

The extension is a landmark development in the history of the gallery, which originally opened on Harcourt St in January 1908, making it the first public gallery in Europe dedicated solely to modern art. It found its permanent home at Charlemont House in 1933 and, following the decision in 2002 to demolish the former National Ballroom, it was finally able to embark on a much needed expansion.

The extension is a remarkable achievement, especially considering the limited space offered by the site and the need to integrate with the architecture of Charlemont House. It is an airy, whitewalled space, with a central atrium drenched in natural light. The transition from the old part of the building to this contemporary space is seamless; there is an easy-to-follow route from room to room; and the architecture does not overwhelm the art in any way.

Now it finally has the scope to host temporary exhibitions. "Before when we put on a major exhibition we had to take down part of our permanent collection. Now we don't have to do that, " says gallery director Barbara Dawson. While the extension will house temporary exhibitions, the older part is dedicated to the permanent collection, with 19th-century continental art and paintings by Irish artists such as John Lavery, Jack B Yeats, Louis Le Brocquy and Patrick Scott on display. The extension also has a bookshop, cafe and children's resource room.

The first temporary exhibition is a retrospective of the internationally renowned conceptual artist Brian O'Doherty/Patrick Ireland. It is a particularly appropriate choice, considering O'Doherty's fame for his groundbreaking collection of essays, Inside the White Cube, which revolutionised the way people think about gallery space. It is also an important coup as it is the Irish artist's first retrospective this side of the Atlantic.

The extension also marks the opening of a purpose-built Sean Scully room. The Dublinborn abstract painter has donated eight paintings to the gallery. Scully worked in collaboration with the architects on the room's design, and the result is an impressive double-height, naturally lit space.

Another important set of acquisitions are six unfinished paintings by Francis Bacon, a noteworthy addition to the Bacon studio.

Five of these were previously on loan to the gallery, while the sixth has been identified as a portrait of Lucien Freud.

Other additions to the collection include paintings by Philip Guston and Ellsworth Kelly, as well as works by Irish artists Gerard Byrne, Jaki Irvine, Brian Maguire and others. And a welcome return will be Harry Clarke's stained glass window of the 'Eve of St Agnes', which has been in storage since 1999.

Last but not least is the stunning exhibition of eight Impressionist paintings from the Sir Hugh Lane Bequest. They are all on display together in Dublin for the first time since 1913 and include such masterpieces as Renoir's 'Les Parapluies', Degas's 'Sur la Plage' and Manet's 'La Musique aux Tuileries'.

Hugh Lane had bought these now worldfamous paintings and 31 others with the aim of establishing a collection of modern art for Ireland. But frustrated by the lack of support from Dublin Corporation in finding a permanent home for them, he altered his will in 1913 so that they would be donated to the National Gallery, London. However, he later added a codicil which bestowed them on Dublin, provided a suitable building was found.

But Lane died tragically when the Lusitania sank in 1915. The legality of the codicil was contested because it had not been witnessed, and ownership of the collection went to London.

After years of disputes, there is now an agreement in which the majority of the 39 paintings are on permanent loan to the Hugh Lane, while the eight most famous paintings rotate in groups of four between the two galleries. Exhibiting all eight together, to celebrate the reopening, is only a temporary arrangement. "This is over and above our agreement, " says Dawson. "Our agreement is of sharing them between us, and at the moment we are very happy with that."

The Lane Bequest has finally come home to the permanent space that Lane had dreamed of. But not for long, so catch them while you can.