National Gallery of Ireland

This gallery has a Caravaggio that was hiding in a Jesuit dining room for decades, and entry is completely free. The National Gallery of Ireland opened in 1864 and houses over 15,000 works spanning European art from 1300 to the present day. The permanent collection includes Vermeer, Rembrandt, Monet, Picasso, and an entire wing devoted to Jack B. Yeats, whose expressionist paintings of Irish life are among the most important works in Irish art history.

But the headline act is Caravaggio's "The Taking of Christ," painted in 1602 and lost for nearly two centuries. It was rediscovered in 1990 by Sergio Benedetti, the gallery's senior conservator, who recognised it hanging in a Jesuit residence in Dublin where it had been attributed to a minor Dutch artist. The painting — depicting the moment Judas betrays Jesus with a kiss — is on indefinite loan from the Jesuits and is worth an estimated €50 million. It's one of the greatest art detective stories of the 20th century.

The gallery owes much of its collection to George Bernard Shaw, who left it a third of his estate, including royalties from his plays. Shaw said the gallery had been his education: "I was able to find in it food for the imagination." The Shaw Room, a reading and study space, is named in his honour. His royalties from Pygmalion and its adaptation My Fair Lady alone generated substantial income for the gallery for decades.

Free admission was a founding principle, and it applies to the permanent collection and most temporary exhibitions. Talks, tours, and the audioguide are also free. In a city where tourist attractions can be expensive, this is an extraordinary gift.