If ever a public office has grown around an individual, it is that of Director of Public Prosecutions. The letters DPP seem to be attached to the name of Eamonn Barnes like a university degree. He is due to retire this year, having reached the age of 65.

He will have spent a quarter of a century in the job, its first, and to date only, incumbent since the office was established in 1975, and he has shaped it.

It was a central aspect of the legislation setting it up that the office was completely independent, not subject to the direction or supervision of any other authority. Mr Barnes has jealously defended this independence, and this has included his adamant opposition to discussing his decisions.

"The job came with a background of politicians phoning sergeants and trying to get them to square a licensing offence or drink-riving charge," a barrister explained. "There's a bit of unease that he has total control and is unanswerable, but I don't think there's any answer to it."

However in his first - and only - annual report, issued earlier this month, Mr Barnes explained the legal and philosophical framework for the functioning of his office.

"The independence of the Director of Public Prosecutions does not exist for its own sake or as a good in itself. Like the independence of a judge, it exists to serve a purpose which is of the essence of the office, the calm, objective and impartial discharge of functions which involve very important issues of justice for those affected by them," he wrote.

This independence is designed to counter the pressure both to drop prosecutions and to prosecute, he said. He specifically addressed the question of a public clamour to initiate certain prosecutions, or to increase the perceived prosecution rate in relation to certain offences. Notwithstanding this, "prosecutorial decisions must always be taken solely on the merits of each individual case."

Early in his career he expanded on this. In an interview with the Irish Press in 1979, he said: "The decisions you take to prosecute somebody are explained in court. You do not offer explanations for your decision.

"What is far more difficult are the decisions not to prosecute and the most difficult decision of all is the decision to withdraw prosecution . . . The decision not to prosecute requires far more courage than a decision to prosecute, and it is a decision taken in isolation without the possibility of explanation."

One of his most controversial decisions was that taken in 1983 not to prosecute Malcolm McArthur, a social acquaintance of the then attorney general, Mr Patrick Connolly, for the murder of Donal Dunne. There were calls in the Dail for an explanation of the decision.

While Mr Barnes refused to give one, he did issue a statement explaining why he felt he could not, and offering some of the reasons such a decision might be made. He pointed out that if he made a statement in this case, he would have to make them in all cases, which could amount to a conviction without trial or to suspicions of malpractice, or both.

He went on: "The reason for non-prosecution often has little or no relevance to the issue of guilt or innocence. It may be, and often is, the non-availability of a particular proof, perhaps purely technical, but nevertheless essential to establish the case. It may be the sudden death or departure abroad of an essential witness."

This issue of explaining decisions is elaborated upon in the annual report, in which he points out that a system of internal review of decisions exists, that if the Commissioner of the Garda Siochana requests a review of a decision it will be carried out, and that such a request from a person having a legitimate interest (such as a victim or a suspect) will be sympathetically received.

Nonetheless, Mr Barnes himself acknowledges that the issue of "transparency and accountability" is raised by this question.

"We all agree that there should be the maximum amount of transparency. But how do you do that without destroying people's lives? It has agitated us for many years. I have discussed it with other people in other jurisdictions. All the ideas mooted have flaws," he told The Irish Times.

He referred The Irish Times to the annual report, which outlines the mechanism which exists for removing him from office, and also describes the interaction with the Garda Siochana and other State offices as one form of accountability.

While the DPP is not, and argues persuasively that he cannot be, accountable for his decisions, there has never been any suggestion that he has abused this power, and his reputation is high among the lawyers who work with him.

"He's grand, he's got very good standards. He has very good people working for him," said one. "He's very well thought of."

"He's a very good head," said another. "He developed the office and it never fell on its head."

This reputation for care, fairness and efficiency makes him a hard act to follow. Lawyers in the criminal area would like to see a successor who, like him, has a cool head and is not a zealot, who might be susceptible to the latest pressure from politicians or the public. Whoever it is will never have a career like that of Mr Barnes - the term is for seven years.

Mr Barnes himself cannot wait to retire, he told The Irish Times. "So many of my friends go into a decline when it comes to retirement. I'm pawing the ground. I would have retired three years ago but I was elected the head of the International Association of Prosecutors for three years."

He is looking forward to reading all the books he has bought over the past 25 years, but never had time to read, and to buying more, especially history. He and his wife are also inveterate travellers, with a particular love of France, where they spent all their family holidays.

"And I'll be doing a bit of legal research - but for its own sake," he added.