`Most people think I'm mean, vicious and horrible. That's only because they see me cross-examining witnesses on television. In reality I'm known in legal circles as a big marshmallow." Barry Scheck devours smoked salmon and brown bread at 10.30 a.m. in a swanky Dublin hotel. He doesn't look either mean or soft. He looks too youthful to be 48, but his polite, long-suffering manner indicates that he has spent far too much time doing tiring, unrewarding things. One of these is interviews, but he puts up with questions patiently. In a nearby cafe, his wife Dorothy (Deedee) and daughter Olivia are eating pancakes, waiting for Dad to get done with the boring part of the day. They want to visit the Joyce tower in Sandycove before Dad has to give his lecture. It's Monday and Scheck is giving the fourth Annual Nissan Lecture later that evening at TCD.

Scheck is best known as the US criminal defence lawyer who represented O.J. Simpson and Louise Woodward. He is currently working on another high profile case: the unsolved case of the murder of beauty pageant girl Jonbenet Ramsay in Boulder, Colorado. "I'm an adviser to the prosecutor, but I won't appear in court because this may result in the death penalty, and I am opposed to the death penalty." He won't say much about the case, except that he has recently been in Boulder to go through the Ramsay's former home to get a perspective on the crime scene evidence the police have already gathered: "It was a terrible crime, and that empty house is really eerie." Scheck is also a professor of law at Cardozo School of Law in New York city, and this is where he tries "to encourage students to change society". He himself decided to go into law because "lawyers played a prominent role in the Civil Rights movement in the early 1960s when I was growing up. They used the law as an instrument of change to better society. To a young, idealistic person, it seemed like a noble profession." He sighs. "Now people see the law as a way to make money."

Scheck teaches an ethics course and in the first class he gives his students three cards to fill out: "One is why did you come to law school, two is why do you think your fellow students came, and three is what is your opinion of the legal profession." The answers, with some exceptions, are usually: one, to change society, two, to make money, and three, greedy, conniving cheaters. He wonders "what sort of socialisation is going on when people start out so idealistic yet the profession is seen in this light?" For him the main functions of the legal profession are "to represent the poor, change institutions and improve society". It is this thinking which led to the Innocence Project which Scheck co-founded in 1992 with his colleague Peter Neufeld and which he spoke about at TCD on Monday night. This is a clinical programme at Cardozo Law School which has either represented or assisted in the representation of 33 men who were exonerated through post-conviction DNA testing and freed from lengthy prison sentences or the death penalty. Scheck was inspired to set up the Innocence Project during the first case he took on with Peter Neufeld: "A man called Marion Coakley was convicted of robbery and sexual assault in the Bronx. He had been at a prayer meeting with 11 other people at the time," yet there were three eye-witnesses who claimed they saw him at the scene of the crime. This, he says, is not as bizarre as it sounds. Why, was the man black? "Yes," responds Scheck, with that long-suffering look. "Yes."

Other evidence came to light: "There was a palm print on the car that did not belong to Coakley. There was a description of the perpetrator as having a Jamaican accent, which Coakley did not have." In the course of that case, he and Neufeld "learned about DNA testing". As a result of the next, similar case they took on, even scientists who were acting as witnesses for the prosecution had to concede that DNA testing had not been carried out correctly. The National Academy of Sciences formed a commission to investigate the matter and a book was published entitled DNA Technology and Forensic Science. The commission recommended that legislation be passed to establish regulatory bodies to look at DNA labs in particular and crime labs in general. "So far only one state has passed this legislation," says Scheck. "Four years ago, it was passed in New York state, and we now have the Forensic Science Review Board." Scheck and Neufeld are commissioners on the board.

"We have worked with the police to set up a DNA data bank which includes DNA profiles from crime scenes of criminals who have not been apprehended, DNA profiles of missing persons, and DNA profiles of convicted murderers and sex offenders," says Scheck. "The data helps you to see where cases which may seem unrelated have actually been carried out by the same person. It is a good way to generate suspects."

He is on his way to visit Dr David Werrett in the UK who runs a similar data bank: "His is more sophisticated than ours," admits Scheck. He also wants to visit the UK's Criminal Case Review Commission which was established as a result of the Birmingham Six and Guildford Four to "evaluate old cases where miscarriage of justice has been claimed: it's the same way we run the Innocence Project except they get state funding," says Scheck, who admits to being "a bad fund-raiser". "What we need is some rich person to just take it on and pay for it. Then we'd be able to get more innocent people out." Scheck and his colleagues do not get paid for their work on the Innocence Project.

Scheck's face is animated now, he has finished his salmon and is pouring himself another cup of tea. "We have to face the fact that the system is imperfect, especially in the US, where so much depends on race and class. We also have pre-trial publicity in the US, which is wisely prohibited over here."

Louise Woodward has said her pre-trial publicity was "atrocious". "Yes, it was terrible; it was particularly mortifying because she knew it could never happen in the UK. Within 24 hours there were screaming headlines saying `Baby Killer'. They had decided she had done it before an autopsy, before an investigation. Even though all the evidence pointed towards an older injury, the jury members said they had a negative impression because of the pre-trial publicity."

He is quick to say that in both the Louise Woodward case and the O.J. Simpson case, he does not know exactly what happened: "In Woodward's case, I know the injury was old so that made her not guilty of the charges. In O.J.'s case, he was acquitted and he has always denied the murder. But there is no way that I can say I know."

He is clearly delighted that Louise Woodward is now free to get on with her own life. He regards her as "a bright and responsible young woman, more than capable of making her own decisions". It may have seemed risky, opting for an all-or-nothing verdict, where the jury was not allowed to consider charges less serious than murder, but he argues "it turned out OK, didn't it?" He explains: "It was Louise's decision. She had our advice, and also the advice of a lawyer we hired who was independent of her defence lawyers. She didn't want the jury to convict her of anything for a crime she didn't commit, and with a charge of manslaughter, she could have got five to 10 years in jail."

Scheck has a particular interest in the peace process in the North ("I'm really hopeful") as a result of having defended various people associated with "the Irish nationalist cause" in the early 1980s. Two of these were Colm and Eamon Meehan, whom he defended without a fee: "Eamon Meehan, a Catholic, had been in the British army and was married to a Protestant girl. He was out demonstrating in the Civil Rights marches and he got caught up when the Troubles ignited. He was accused of killing someone on the day of his wedding. Both he and Colm were tortured. Colm subsequently won his case at the European Court of Human Rights. They both fled to the US, but they were broken, physically and spiritually." The two were asked to assist in packing a container with guns destined for Ireland. They were convicted but not deported: "The judge took pity and allowed them to stay in the US, keep their green cards and stay with their families." Scheck was clearly moved by the case of Eamon Meehan: "I got close to the situation. After all, I had been out demonstrating for Civil Rights too, and against the war in Vietnam. We were the same age; we liked the same music. You can see the tragedies that occur and shatter people's lives."

He has the same expression of intense empathy on his face when he talks about the cases of police brutality in New York which are his current major concern. He describes the case of Abner Louima, mistakenly arrested outside a Haitian nightclub: "He was beaten up on the way to the precinct. When they got him there they took him into the bathroom and stuck a baton up his rectum, and perforated it. They then shoved the baton, covered in blood and faeces, into his mouth, uttering racial slurs all the time. They have been indicted." Scheck does not come of lawyer stock - his Jewish father was a tap dancer and then a musical agent. Born in Manhattan and educated at Yale and the University of California at Berkeley, Scheck hasn't moved far from his roots: he now lives at Fulton's Landing in Brooklyn. His son is attending Brown University, Rhode Island, and wants to follow his father's footsteps into the law. "I don't know if it's such a good idea," says Scheck, shaking his head and smiling ruefully. So, as one of American's best known lawyers and a committed teacher, what does he think are the vital ingredients that go to make up a good lawyer? "You need a passion for justice, courage and dedication to principle. You also need to know what you are talking about; anyone can talk a good game, but the real key is preparing yourself properly."

And off he goes, to get a taste of Dublin with his wife and daughter before work swallows him once again.