The True Amazon Adventures of Roger Casement by Andrew Shaw, directed by Robert Reid. With Mike McEvoy, Elliot Summers, Robert Lloyd, Michael F. Cahill, Tobias Manderson-Galvin, Johannes Scherpenhuizen, Liz McColl, Simon Morrison-Baldwin and Alicia Benn Lawler. La Mama until February 18.

The so-called Black Diaries of Roger Casement are a kind of Turin Shroud of modern history. Sir Roger Casement was a distinguished Victorian human rights advocate whose reports on colonial atrocities in the Belgian Congo and rubber plantations in Peru earned him a knighthood.

But Casement, an Anglo-Irishman, was also a believer in Irish Independence. In 1916 he arranged for a German ship to sail for Ireland with "several machine-guns, 20,000 rifles and a million rounds of ammunition" for the Irish Volunteers. But his plans were exposed, and Casement was arrested and imprisoned in London for three months awaiting trial. He was hanged by the British for treason on August 3, 1916, for his part in the Easter Rising.

While Casement was in prison, the diaries - supposedly seized in a raid on his house - were used to destroy his credibility and character. The diaries contained explicit details that revealed Casement to be a promiscuous homosexual with a taste for rough trade. Selected extracts were shown to public figures and known sympathisers, who consequently shrank back from appealing for clemency for a "degenerate". The Black Diaries effectively hanged him.

Predictably, perhaps, given the underhand way in which the British authorities used the diaries, and their subsequent keeping in conditions of extraordinary secrecy (the first independent examination of the documents only happened three years ago), their provenance has always been surrounded by controversy. Particularly in Ireland, there has been a widely held belief that the diaries were forgeries, partly out of a disbelief that a hero and martyr could possibly be gay: as de Valera said, he was "too noble to be a degenerate".

The unequivocal 2002 judgment of handwriting expert Dr Audrey Giles that the diaries "were genuine throughout and in each instance" has done nothing, however, to end the controversy. Many experts argue that her examination was incomplete, and failed to take into account inconsistencies in the text and other issues which, at the very least, throw doubt on their authenticity.

In our times, Casement is in danger of becoming a martyr for gay pride as much as for the Irish Nationalists. Poor ghost. Playwright Andrew Shaw has no doubt: "we can accept the diaries as real, why shouldn't they be?" he says, dismissing the arguments for their inauthenticity as "a claim designed to safeguard an Irish martyr against the perversion of homosexuality".

I'm not so sure; in this age, the counter-arguments may have nothing to do with homophobia, and everything to do with concern about the lengths to which the British authorities could go in order to hang a troublesome dissident. However, Andrew Shaw has created an intelligent and witty play out of the hallucinatory realities that circle around this case.

In the opening scenes a young civil servant Thomson (Mike McEvoy) is blackmailed by two Foreign Office officials (Robert Lloyd and Michael F Cahill); the police have certain information on his private life, and he will be prosecuted for homosexuality - unless, that is, he reads the private diaries of Roger Casement and uses them to create a forgery which fits in with the known details, but which proves Casement to be a degenerate.

Thomson likes to think he is a humanitarian - he admires Casement's work in the Congo and the Amazon - and is something of a naive romantic. But he takes the job to save his own skin, knowing that he will help to hang a man whom he admires. He hopes to salve his conscience by showing that, even though Casement is queer, he is also a human being capable of love; and the project also appeals to a certain literary vanity. The irony is that when the actual diaries turn up and Thomson's forgeries are no longer needed, Casement's adventures are not the romantic idylls of Thomson's imaginings, but something altogether more ambiguous and disturbing.

Shaw interleaves scenes between the civil servants and others between Thomson and his lover with dramatisations of events from Casement's diaries, which relate a somewhat brutal narrative of what we would now call sex tourism as well as the corruptions and brutalities of plantation life. He artfully illustrates not only the hypocrisies of Victorian society - at least one of Thomson's superiors is himself homosexual - but also its mechanics: the levers of class and money and exploitation that constitute a colonial empire. Sharp and subtly inflected performances from these three actors (I especially enjoyed the panicked vulnerability of Thomson, too intelligent to hide from himself the implications of what he is doing) intensify the ironies of these scenes.

In the middle of this machinery is the hapless character of Casement (Elliot Summers) himself, who is a cipher - on the one hand condemning the exploitation of "natives", while on the other exploiting them sexually. But instead of creating a truly complex contradiction, Casement comes across merely as a hypocritical prig, weakly shoring up his own authority at the expense of the man he claims to love but, in fact, exploits and betrays. It's a factor of Summers' rather blank performance, I think, as much as a question not quite resolved in the script. While Shaw's Casement is certainly flawed, it is difficult to see how such a moral quisling could be fired by the desire for justice that motivated his reports of human rights abuses or the support for an independent Ireland which finally ended his life.

Robert Reid's direction is arresting, if perhaps a little ambitious for the intimate stage of La Mama: the artifice of this production might work better with the distance of a proscenium. The actors are in white face, and half masks are used to indicate the masks of colonial rule, effectively theatricalising the roles and selves of colonial rule. The English servant and the plantation slave, both at the bottom of the class hierarchy, are represented by a bunraku puppet. Sometimes the staging is very effective indeed: a scene where Casement's assistant is whipped is startlingly violent and unbearable, especially from a distance of less than a metre - hard to do in a small theatre. It's a production that doesn't quite achieve its own ambition, but is well worth a look.