A documentary on Roger Casement includes new evidence that the Black Diaries, recording Casement's homosexual experiences, were authentic. If true, this makes his complex legacy even more fascinating, says the programme's maker, Alan Gilsenan.

The Black Diaries, explicitly recording Roger Casement's homosexual experiences over the course of several years, were used by British intelligence services to smear the Irish nationalist leader hanged for treason in 1916. That much is agreed by all sides. But are the diaries authentic? Tomorrow night, the second part of Alan Gilsenan's documentary, The Ghost of Roger Casement, will be shown on RTÉ1. At the centre of Gilsenan's film is yesterday's announcement that comparative analysis of the handwriting in the Black Diaries with Casement's other writings provides "overwhelming" evidence that the Diaries are genuine.

The report from the London-based Giles Document Laboratory, run by Dr Audrey Giles, was commissioned by Prof Bill McCormack of Goldsmiths College and part-funded by RTÉ and the BBC. The impetus for a scientific analysis of the Black Diaries grew out of the Royal Irish Academy's symposium on Casement in spring of 2000, an event which Gilsenan had filmed for his documentary.

"There was a feeling then that the subject needed to be put to bed, and that a major international forensic investigation would be the way to do it," says Gilsenan. "But there is a lot of new scholarly writing about Casement due to come out next year, and for that reason the Government backed away from having a forensic investigation immediately.

"Then Prof McCormack, on his own initiative, decided he would run with an investigation. Obviously, in the context of a documentary, one would be foolish not to film that. It's gone from being a historical documentary to being a news piece, and that's very attractive, to see living history being made. But from my perspective, this film is firstly about Casement's extraordinary life, and only secondly about the forensic investigation."

Born in Antrim, but raised largely in England, Roger Casement rose to public prominence following his reports on Belgian atrocities in the Congo and the abuse of rubber plantation workers in South America, which established him as one of the most important humanitarian figures of his time. Although knighted in recognition of his work, he was already becoming more and more convinced of the need for Irish independence from British rule, a belief which brought him first to the US on the eve of the first World War to rally support in Irish-American communities, and then to Germany, where his attempts to recruit Irish prisoners of war as volunteers against the British ended disastrously and ignominiously.

Landing on the Co Kerry coast on the eve of the Easter Rising, he was quickly arrested and brought to London, where he was tried for treason. The discovery of the Black Diaries in a London flat provided a convenient - too convenient, some say - means for the British government to discredit Casement and undermine the international campaign for clemency to be granted.

"Declan Kiberd said in an interview that didn't make it into the final film that the thing about Casement is he disrupts all codes," says Gilsenan. "He's kind of Irish but not Irish; kind of Catholic but not Catholic; kind of straight but not straight. He brings up an awful lot of issues in Anglo-Irish history. Having spent some time on it and talked to both sides, it's a very complex dynamic."

It's this sort of ambiguity, arguably, which makes Casement such a fascinating and attractive figure in the current age. As a homosexual, a human rights campaigner, an anti-racist and an anti-imperialist, he chimes better with some 21st-century versions of Irish nationalism than do some of his contemporaries. Statements, such as one at the RIA symposium that "if these diaries are true, then Roger Casement cannot be an Irish nationalist", tend to reinforce the belief that the forgery thesis is the province of backwoodsmen, but Gilsenan doesn't see it that way.

"There has been a belief, an understandable one, that all the forgery thesis people are fringe lunatics," he says. "But, having made this documentary, that wouldn't be my feeling. At times they haven't made their case in the best way possible, but they do have a case. I don't think it's a black and white issue."

Therefore, Gilsenan, who provides the commentary for the film himself, makes clear that he accepts that the available evidence points to the genuineness of the diaries, while he also gives space to those who continue to reject them.

It can hardly be denied, though, that the forgery theory relies on evidence which is circumstantial at best. The strongest proponent of the theory in The Ghost of Roger Casement is the Scottish historian Angus Mitchell, an acknowledged expert on Casement's writings.

"I repeatedly asked people, particularly Angus Mitchell, who I have a lot of respect for, to tell me something that would convince the people of Ireland that these diaries are forged," says Gilsenan. "The answer was invariably the same: it is in the detail of Casement's writing and in the historical analysis of his legacy that one can see the inconsistencies. I think there is some validity in that, and I think those people are honourable in their intentions.

"One has to remember that there's an enormous body of Casement's writings which has barely been read. To be fair to Angus Mitchell, who takes an enormous amount of flak for his position on the side of the forgery thesis, he is the one person who is wading through that material in some detail."

While its conclusions are clear and unambiguous, there may be some disappointment with the extent of the research undertaken on the diaries, Gilsenan believes. The investigation was presented in advance as a "forensic" one, but Dr Giles's conclusions are based exclusively on her analysis of the handwriting. The steering committee has commissioned position papers on other potential tests - of paper, pollen traces or DNA - but there seems to be little confidence that any of these will yield definitive results.

"One of the unfortunate things about the forensic investigation is that it wasn't as conclusive as it might have been," says Gilsenan. "It was conclusive in terms of handwriting, but I think what was needed was a very conclusive and watertight investigation. It would have been easy for me to puff up the forensics report in the documentary, to say: 'This is it. We have the news, and the news is Casement was gay and the Black Diaries are authentic.' I felt very strongly that we would be belittling the complexity of the story so, as you see in the documentary, there are reservations about the forensics."

SOME of those who believe the diaries are forgeries have already questioned the independence of the investigation, pointing to Dr Giles's previous background working for Scotland Yard. But her investigation was supervised by a six-person independent committee of English, Irish and Scottish experts. Surely it strains credulity beyond breaking point to suggest that some continuing, century-long conspiracy might affect the conclusions of this extremely public report?

"I worked with Audrey Giles and I accept her credentials totally," says Gilsenan. "But the fact is, and this is separate from her credibility and background, that this is an incredibly emotional, controversial issue. If you're going to do an investigation into something which provokes so many emotions and doubts and conspiracy theories, then you need to do something that's watertight."

One of the things he realised while making the documentary, he says, is that this particular history isn't as far away as it might seem. "Casement's time is not that far removed from our own. And if there are questions about a conspiracy by British intelligence and by Scotland Yard less than a hundred years ago, one has to ask whether it's advisable to have somebody who worked for that same establishment, and wasimplicitly part of it, to be the arbiter on whether the diaries are real or not.

"From what I've seen and known of Audrey Giles, she is a woman who has been involved in forensic cases on every side of the political divide, and indeed on every side of the Irish question. So I would certainly respect her independence. But I have to say that I think there are valid questions as to why we didn't get a Swiss or a Danish examiner. If you're going to debate a conspiracy theory, you should be able to deal with those kinds of fundamentals."

It seems, therefore, that, despite its apparent clarity, this new report will not cause many people to change their views. "The people involved in the debate will probably continue to debate it," agrees Gilsenan.

"But I think there was certainly an ambiguity in the public imagination about the diaries, and I hope the documentary will put that to bed, and that we can move on and start to understand Casement as a world figure and a nationalist figure in the broadest sense. There's also something energising about the Black Diaries and the way they put Casement on the agenda.

"It's incredibly exciting to have something that starts as history and ends as news. It resonates down through time."

The Ghost of Roger Casement is on RTÉ1 tomorrow at 10.10 p.m.