PUSHING through the hubbub outside Paddington Green police station on Wednesday, the human rights lawyer, Gareth Peirce, may well have felt a sense of deja vu. Having built her career exposing the wrongful treatment of terrorist suspects by the police, including the Birmingham Six and the Guildford Four, she was in no mood to accept the Met's detention of the Guantanamo Four. Her irritation showed when she spoke to reporters outside.

Her clients, Asif Iqbal and Shafiq Rasul, were suffering ''severe sleep deprivation'', she thundered, and it was ''very clear that their cells were too cold''. She accused the British police, furthermore, of ''compounding'' the injustice suffered by the men during two years' imprisonment. Inside, the police were listening. By the time the four men were released later that day, the officers may well have felt glad to see them go, not to mention their redoubtable solicitor.

These days, no-one underestimates the thin, quiet woman in the unmemorable suit. Credited with enormous persistence and unwavering attention to detail, Gareth Peirce is a legal hurricane. The claim of her one-time boss, Benedict Birnberg, that she has ''transformed the criminal justice scene in this country almost single-handedly'', might sound like hyperbole, but a glance through her past caseload bears it out.

Her presence, physically unobtrusive but intellectually dominating, has been a feature of nearly every big miscarriage of justice case of the past 20 years, especially those involving Irish nationals. It was Peirce who acted for Gerry Conlon of the Guildford Four in the 1989 appeal that led to the quashing of the group's 15-year-old conviction for bombings in Guildford and Woolwich. In the 1993 film about the case, In the Name of the Father, Peirce was portrayed by Emma Thompson.

Two years later, Peirce acted for five of the Birmingham Six, who were wrongfully convicted of the 1974 Birmingham pub bombings. Peirce also took on the case of the so-called M62 bomber, Judith Ward, whose murder conviction relating to 12 deaths in the 1974 bombing of an army coach near Leeds was overturned in 1992. The Winchester Three? Gareth Peirce. The Maguire Seven? Peirce again.

''There is a sad and sorry history in this country of dealing in a particular way with Irish defendants,'' said Peirce in April 1990, the day the Winchester Three, in prison for conspiring to murder the Northern Ireland secretary, Tom King, had their convictions quashed. That Peirce has always been so ready to act on their behalf has led to speculation about her political sympathies, but Birnberg has shrugged it off, believing that she has no hidden agenda ''except to right injustice''.

Certainly, in recent years, she has cast her net wider. In 1996, she was instrumental in the freeing of Sara Thornton, who was convicted of murder for the killing of her abusive, alcoholic husband: that was changed to manslaughter at retrial. She has campaigned on behalf of Samar Alami and Jawad Botmeh, whom she believes were wrongly convicted of the 1994 London bombings of the Israeli embassy and a Jewish charity. She has represented three men of North African descent charged with possessing articles for the preparation, instigation and commission of terrorism; and she agreed to act for eight Britons who were refused entry to Italy where the G8 summit was taking place in 2001.

Indeed, if there is a common thread linking many of her cases, it is a vigilance about police methods and procedure. One of the grounds of the Birmingham Six appeal was that new electronic tests showed the police interview notes could have been written months after the police said they were. The Guildford Four's appeal followed a police investigation that found serious flaws in the way Surrey police had noted the four's confessions.

The case that launched her career - that of the Luton post office murder - gave her an early lesson in the limits of justice. She had recently come to Birnberg's firm in London after an education at Cheltenham Ladies' College and Oxford University, and a stint in the United States where she had been involved with the civil rights movement. She was married to an American, Bill, and had one young son. She had not started her formal training with Birnberg at the time, 1974, but he nevertheless asked the promising young lawyer to prepare the case, which was going to appeal for the second time.

It focused on two men, David Cooper and Michael McMahon, who were supposedly part of a four-man gang that had shot dead a postmaster during a botched robbery. The original case hinged on the evidence of one of their supposed accomplices, Alfred Matthews, who already had a conviction for post office robbery. He picked Cooper and McMahon and a third man, Patrick Murphy, out of a police line-up as his accomplices and agreed to testify against them if charges against him were dropped. They were convicted.

During the first appeal, fresh evidence of an alibi for Murphy was brought forward and accepted. His conviction was quashed. Yet, even though the judge had accepted that Matthews must have been wrong in thinking Murphy was in his gang, he did not accept that that undermined Matthews's identification of Cooper and McMahon. Many thought the decision highly illogical.

Peirce, with typical commitment, worked ceaselessly on the case while pregnant with her second son, including during labour when she continued to pore over case notes. The appeal, however, came to nothing, an experience she has described as ''traumatic''. There were three subsequent appeals, but still the court would not budge. By then, however, largely due to Peirce's efforts, the case had become a cause celebre. She had recruited the aid of a former law lord to help her with the case and his

masterful exposition of it in public added momentum to the movement. Doubts focused on the credibility and reliability of prosecution witnesses and the officer leading the murder inquiry. In 1980, amid growing public unease, the home secretary remitted the rest of the men's sentences. They died in the 1990s, both just in their 50s, having never had their convictions formally overturned.

''It never feels like a triumph. All it feels is a cessation of failure,'' Peirce was quoted as saying in the Guardian last year, describing what it feels like when the wrongfully convicted are finally released from prison. Indeed, in a mark of how much she honoured McMahon and Cooper's right to justice, Peirce returned to court last year for a sixth appeal: the men's convictions were finally quashed, 23 years too late.

This passion for her cause, according to her colleagues, gets to the heart of who she is. As a solicitor who instructs the real performer, the barrister, she has never relished the attention her work and personal conviction inevitably attract. She is said to have been distinctly uneasy about being portrayed on the big screen. Her very appearance is studiedly ordinary. She also asked the government to withdraw her CBE award offered in 1999, feeling, probably, that it might compromise her work.

Those who have worked with her, including the biggest stars of the Old Bailey (Michael Mansfield, Anthony Scrivener) find her passion compelling, her knowledge of the law unparalleled. Her primary concern, in the case of the Guantanamo Bay detainees, as with all her other charges, is for their well-being. She is quick to speak up to improve the conditions in which her clients are held in detention and she has had some, including Gerry Conlon, to stay in her north London home on their release.

British politics and its judicial system may have changed since the convictions of the Guildford Four and Birmingham Six, but with new far-reaching anti-terrorism laws in place, Peirce can see the dangers more clearly than most. The Prevention of Terrorism Act, which was focused on Irish organisations and extended police powers of detention, had a ''dramatic and chilling effect'' on the Irish community, she told Race and Class magazine in October 2001, adding that that experience was being replicated now among the refugee community. ''The 2000 Terrorism Act is probably the most far-reaching criminal legislation that there has ever been in terms of the scope and breadth of its definitions and its potential to affect far more people than previous anti-terrorism acts,'' she has said. Such legislation, no doubt to her regret, seems likely to keep her in work for years to come.