IN HIS closing arguments in the trial of Brendan O'Donnell, Patrick MacEntee SC offered an eloquent defence, not just of his client, but in a sense of himself.

As one of the greatest defence lawyers this State has seen, he knew that his task was not a popular one. His job was to protect the interests of a man who admitted killing, in cold blood, a tiny child, a beautiful young mother and a much loved priest. His client had laughed in the witness box as he described the killing of Father Joe Walsh.

But in doing that job, Mr MacEntee also provided a ringing justification of the expenditure of his own prodigious talent and energy on the protection of a very bad man.

Justice, he told the jury, was easy to dispense to the well behaved. But it was precisely "the sick, unattractive, nasty, badly damaged" who most needed justice. And Brendan O'Donnell was a "sad, sick, not terribly honest, and singularly unattractive young man".

The fate of such people at the hands of the courts defined the quality of Irish justice. "The Brendan O'Donnells of this world," he said, "are the ones who test the system."

Testing the system is what Patrick MacEntee does incomparably well. For over 35 years now, he has been probing its darkest corners and exposing its deepest inadequacies. He belongs to a long tradition of seeing the legal system as hard won and always endangered. His is not the Olympian view of the law as a set of eternal verities, but rather one that fears that the system, without constant testing, will become "case hardened, cynical, or corrupted with power".

This week, even though he lost the argument in the O'Donnell case, he vindicated his own life's work yet again. For, whatever the verdict, the system provided a despised young man with blood on his hands, not just with a fair trial, but with as good a defence as could be imagined anywhere in the world.

Such cases are the ones Patrick MacEntee will be remembered for. Although he has often acted in civil litigation cases, and could certainly have made a large fortune as a commercial or civil lawyer, he has chosen for the most part to concentrate on the less lucrative but more dramatic role of defence counsel.

That he has been supremely good at it is obvious even from a cursory list of some of the cases he has acted in the Sallins train robbery, the Don Tidey kidnap, the murder of Lord Mountbatten, the Stardust inquiry, the Malcolm McArthur case, the supergrass trials in the North, the extraditions of Dominic McGlinchey and Robert Trimbole, the judicial inquiry into the convictions of the Guildford Four, the beef tribunal and the subsequent Rathkeale trials.

His clients have ranged from multiple murderers to gardai, and from IRA men to the Catholic Church, for which he acted as legal adviser to the Irish bishops advisory committee on child sexual abuse.

It is a role that requires an extraordinary degree of detachment, and Patrick MacEntee's ability to separate private feelings from professional role seems total. He even agreed to defend those charged with the IRA murder of Senator Billy Fox of Fine Gael in 1974, even though be himself had acted as Fox's election agent in the late 1950s.

It is also a lonely role. The leading American defence lawyer, F. Lee Bailey, compares it to being a pilot on a combat mission, and suggests that would be defence counsel should first learn to fly: "The ones who survived would understand the meaning of Patrick MacEntee, who has never been among the Law Library's more active socialites, seems comfortable with the loneliness.

That comfort comes from what seems to be an innate self confidence. Born in Monaghan town 58 years ago, the son of a dentist, he is very much a product of the small town, professional middle class that produced and sustained the Cumann na nGaedheal tradition in Irish public life.

Although he has avoided political entanglements since he became a barrister in 1960, he represents much of what is best in that tradition's self image - a scrupulous, uncompromising adherence to the institutions of law and democracy.

His courtroom style is a distinctive mixture of the 19th and 20th centuries. On the one hand, he is recognisably in the tradition of Redmondite rhetoric, the stately oratory that Fine Gael inherited from the old Irish Party by way of James Dillon and Paddy Lindsay.

Something of his background in the provincial professional class that dominated Fine Gael in the 1950s is still discernible in his deep, rolling voice and classical cadences. The theatricality inherent in a man who spent much of his time at UCD in the 1950s acting in and directing plays for Dramsoc is never far beneath the surface.

On the other hand, the oratory is always underpinned by a rigorous, almost scientific logic that belongs to a different world altogether. Unlike some of his barrister colleagues of the same generation, his performances are never self indulgent, always at the service of a meticulous construction of a case. The actor is kept in check by the technician.

He is not unlike a great musician playing a time worn classic, taking ideas so long held that they are in danger of becoming cliches - trial by jury, innocence until proven guilty, due process and using them as if they had just that moment been invented.

He takes law and order seriously enough to oppose any encroachment on its basic principles, as he has consistently done with efforts to abolish trial by jury, whether for terrorist cases or for insurance claims. He is, in that sense, a conservative, but one whose passion for first principles often places him in opposition to the establishment.

It could be argued, indeed, that his skills played a critical role in preserving the integrity of the legal system under pressure from armed subversion, and the state response to it, in the 1970s.

His long association with the defence of those charged with terrorist related offences is in no way indicative of his personal political opinions.

Indeed, when he became the first Senior Counsel from the South to become a Queen's Counsel in the North in 1985, he had no obvious difficulty in taking the necessary oath of allegiance to the British monarch, and neither that nor his willingness to act for IRA members should be interpreted as more than a professional decision.

But that detachment made the tenacity and courage with which he fought the abuse of Garda powers in the State's campaign against republican subversion all the more remarkable. More than anyone else, he broke through the implicit faith of the courts, and the wider society, in Garda evidence, and forced the legal system to recognise that abuses could take place.

In a series of cases, he effectively made new law a somewhat paradoxical achievement for a man who has always been highly critical of the way Irish courts have effectively taken on some of the law making functions of the Oireachtas.

In testing the Offences Against the State Act, he established that suspects had to be told when their period of lawful detention had ended, that they had a right to legal advice, and that they could not be held for more than one seven day period of detention in succession.

It has long been expected that ability to change the direction of a piece of law would find its natural fulfilment on the bench, but the fact that he did not seek one of the recent judicial appointments may indicate that he intends to end his career in the role to which it has added lustre, that of the great defender.