The Irish poet George Russell called George Bernard Shaw "a suffering, sensitive soul" - a point perfectly proved by this rarely seen play dating from 1904. Beneath its satire on Anglo-Saxon and Irish attitudes and its assault on entrepreneurial capitalism lies a deep vein of grief that is quintessentially Shavian.
In truth, Shaw's robust jocularity is sometimes a bit heavy-handed. We watch, with polite amusement, as two civil engineers descend on the rural community of Rosscullen to set up an idealised garden city. Broadbent, the bluff Englishman, turns out to be a closet romantic who proposes to the local heiress and offers himself as a Home Rule parliamentary candidate. His partner Doyle, an Anglicised Irishman, laments his fellow-countrymen's irresolute dreaming and victim culture. All this is good paradoxical fun, but a touch dated in the age of the Emerald Tiger and Ireland's embrace of the single currency.
The heart of Shaw's play lies in the extraordinary character of Peter Keegan, a defrocked priest who sees the world as a place of "torment and penance" and uses feigned madness as a mask for profound truths. Keegan strikes me as a creation of genius in that he combines estrangement from the world with a devastating economic critique. At the play's haunting, elegiac close, he unpicks capitalism's "foolish dream of efficiency" and the consequences of buccaneering enterprise while painting a vision of a perfect universe that brings a lump to the throat.
Long characterised as a didactic brainbox, Shaw, at his best, was a displaced poet. This is shown in Dominic Dromgoole's production by the use of Wagnerian motifs and by Niall Buggy's transcendent performance as Keegan. With his lilting tones and boyish, well-scrubbed features, Buggy suggests a tragic saint who is ill at ease in a world he sees as a choice between hell and purgatory. But when he impartially attacks the Englishman - "so clever in your foolishness and this Irishman so foolish in his cleverness" - you realise that the character of the visionary outsider is a revealing Shavian self-portrait.
Charles Edwards sensibly plays Broadbent as a handsome dasher rather than a Punch caricature. Gerrard McArthur lends Doyle a fine feverish neurosis, and there is good support from Catherine Walker as the prized heiress and John Dougall as a fake Irishman who has learned his lingo from the halls. What makes the play endure, though, is less Shaw's satiric puncturing of Anglo-Irish mutual misconceptions than his Beckettian empathy with human suffering.
· Until October 25. Box office: 020-7328 1000.