This is an important and powerful but also - at times - a seriously flawed book. It is important because it is the first full-length study of a prelate who was arguably one of the most significant figures in 20th-century Ireland. It is powerful because it is largely based on original research in the McQuaid archives and elsewhere which throws a fascinating new light on the man and his times.

It is flawed by analytical and evidential weaknesses which generate glib and sometimes cliched conclusions or inferences about complex social, political and personal matters: these do a disservice both to the author and to his subject.

Nobody who has an interest in the history of modern Ireland will read it without being enthralled by the windows it opens into hitherto unseen corners of our national life. The archives are rich indeed, and John Cooney has gone through them with a searchlight. In many areas, a telling phrase is picked out to illuminate an aspect of McQuaid's character, or of his interaction with others, in a way which adds immeasurably to our understanding of the ways in which the struggle for power was waged, in the old days, behind closed doors.

The story he tells is intriguing by any standards. The intellectually gifted child of a country doctor, whose mother died when he was an infant but who did not become aware of this until his teenage years, McQuaid was a brilliant and dedicated student from his early years, typical in a way of the middle-class child destined for the priesthood. (He was also gifted in other ways if, as Cooney says, he could shoot snipe with a rifle.)

For many years, though, his ambition was not ecclesiastical preferment, but missionary service: at least four requests to be transferred to Africa were turned down by his superiors. He could have been one of the greatest missionary bishops of the century - all that energy, and intellect, would have gone through the continent like a whirlwind.

These talents were unleashed instead on Dublin and on Ireland. And the social and political context in which McQuaid became archbishop was quite extraordinarily malleable. It was a Yeatsian scenario in which the worst were full of passionate intensity and the "yahoo laity", as Peadar O'Donnell tellingly described them, were in full cry against the unorthodox. McQuaid had a ready-made army at his command, and he marshalled it like a Napoleon to sniff out heresy and crush dissent.

It is not an edifying picture. If one episode sticks in the memory more than anything else, it is Cooney's account of McQuaid's demolition of the Mercier Society, a discussion-group of lay Catholics - Frank Duff among them - which tried to initiate meaningful dialogue with Protestants and Jews. Duff is currently being promoted for sainthood. On the evidence of this book, another Mercier Society member, Leon O Broin, is a far more deserving candidate.

The Catholic hierarchy, bereft of anything that might be described as leadership, was equally easy prey. McQuaid filled the power vacuum effortlessly, negotiating with governments and ministers in a style which blended realpolitik, moral imperialism and old-fashioned bullying. Cooney's account, based on McQuaid's extraordinarily detailed memoranda, of the way in which he masterminded the defeat of Noel Browne's Mother and Child scheme, will make the hair stand up on the back of your neck. My own feeling is that the zenith of McQuaid's power came not in 1961 as Cooney suggests, but rather earlier. Even in the middle and late 1950s, while official Ireland was still bending the knee, his legislative and other victories were beginning to have a somewhat hollow ring to them. More and more people were beginning not to take notice, or to care, although they wisely kept these sentiments to themselves. Certainly the 1960s were traumatic for someone of McQuaid's background and experience; and there is a genuine pathos about the final years.

The problems posed by the book fall into two areas. The first is the way in which the author categorises the social and political context of the times. These judgements sometimes read more like 1960s journalism or a prosecution brief: this is not a benefit-of-the-doubt biography. And Cooney's own certainties sometimes lead him into overstatement, or make him put odd constructions on actual texts. Costello's subservience to the Catholic Church on matters of faith and morals, for example, is hardly revealed for the first time by the McQuaid files. It was revealed fairly unambiguously by the man himself in 1951, but it was not remarkable at the time, except by malcontents and misfits, who could safely be disregarded.

Similarly, McQuaid's interventions with de Valera (who, on the whole, comes out of this rather well) on the draft texts of the Constitution do not bear out Cooney's argument that the archbishop was responsible for Article 3's contentious - to Unionists - phraseology. The most controversial phrase in that article came from de Valera, not Drumcondra. And, ironically, McQuaid's preferred wording for the divorce section - that divorce should be forbidden for baptised persons - was more restrictive, though less workable, than de Valera's formulation, and actually prefigured the report of the 1967 All-Party Committee on the Constitution, which was roundly condemned by McQuaid when it appeared!

More seriously, there is a sub-theme to the book which is very contemporary in tone but for which the evidence is dangerously weak. It is partly the Browne manuscript allegation, but this is buttressed by a number of other passages. These include interviews with two people who, as boys, felt some inappropriate sexual vibrations in McQuaid's dealings with them; the late Mercy Simms' opinion; the allegation that one of McQuaid's henchmen in UCD favoured young students of "a Grecian appearance"; McQuaid's own, guarded reference to his schooldays; a statement that McQuaid was seriously beaten by his own father as a child; and, as serious as any, an allegation that McQuaid and Gerald Boland, then Minister for Justice, collaborated to squash a Garda investigation into sexual abuse at a Dublin institution.

THE cumulative effect of these allegations is to imply the existence of a clerical satyricon in which the most vulnerable children in our society were helpless because corruption stretched all the way to the top. It is an appalling vista - if true. The problem is that we cannot determine its truth on the evidence provided. McQuaid's reference to his own schooldays is the only hard evidence, and it's not evidence of much. The alleged beatings by his father (which surely did not post-date 1910, at the latest) are credited to an unnamed, but presumably long-lived, medical source. The two former schoolboys are unidentified but, even if they were named, their evidence is hardly compelling. The reference to the UCD figure is not even dignified with a source, and is the sort of malodorous innuendo familiar to anyone in academic life. Browne's accusation, although evidently believed by Browne, will be conclusive evidence only for a relatively small number of the faithful. And the major allegation about frustrating a Garda investigation is unverifiable because, again, it is credited to an anonymous source.

Anonymous sources are useful - sometimes essential - in journalism, even though they are used by journalists far more than they should be. In a work of scholarship they should be used much more sparingly and, where possible, to illustrate incidentals, rather than to support major findings. Here, they are a series of large holes below the water-line of credibility, and self-inflicted ones at that.