Beware the author's note in novels: If the author starts confessing and apologizing before the Roman numerals have disappeared from the tops of the pages, there may be trouble ahead.

In "Cathedral," author Nelson De Mille tells us in his note that he has taken "dramatic liberties" and has also "learned that in any book dealing with the Irish, literary license and other liberties should be not only tolerated but expected." De Mille, unfortunately, takes too much liberty and not enough care.

"Cathedral" is the story of a renegade group of former Provisional Irish Republican Army members, calling themselves "The Fenians," who take over St. Patrick's Cathedral in New York on St. Patrick's Day, sometime in the future. They hold four people hostage in the cathedral: New York's cardinal, his faithful priest companion, the British consul general and a woman who is a former IRA member. The Fenians threaten to kill the hostages and blow up the church by 6:03 the next morning if 137 prisoners in jail in Britain and Northern Ireland aren't "released by sunrise." They also want immunity from prosecution, transportation to Kennedy Airport, a laissez-passer and 45 corned beef and cabbage dinners.

The demands, however, are not very important. In fact, they are hardly ever mentioned. What is important here is "the human drama": the interaction between the police (especially Lt. Patrick Burke, an Irish American) and the terrorists, between lover and ex-lover, between heroes and lunatics.

The novel's lunatics come mostly from the ranks of the Fenians, whose invention by De Mille represents the loosest of his "liberties." The real IRA, as De Mille himself tells us, maintains "a policy of hands-off America." And as an organization with strong Catholic roots, the IRA would be unlikely to take over a cathedral and hold a cardinal hostage. Thus a fake IRA is necessary if this "what if" novel is to get off the ground.

There's nothing wrong with this approach if it works. But, in the case of "Cathedral," it doesn't. Right down the line, the Fenians are crazed, one-dimensional, bloodthirsty maniacs. These so-called Irish rebels reveal no genuine passion or substance. De Mille rarely allows any of them to offer any kind of articulate justification for their actions. Instead, there is a brief debate, a few platitudes here and there, and one ridiculous rant-and-rave session before the television news cameras by Fenian John Hickey, a malevolent octogenarian of comic-book proportions.

Sometimes major conceptual failures, like De Mille's creation of the Fenians, can seem less irritating than minor inaccuracies and shortcomings, which tend to reveal a lack of attention to detail, a lack of caring about those particulars that novels need to come to life. If you loved the literary sensibility of Thomas Flanagan's "The Year of the French," or were swept away by the pulpy grandeur of "Trinity," or thrilled to James Carroll's "Madonna Red" and Ambrose Clancy's "Blind Pilot" ("Cathedral" shows the influence of these last two), prepare to be disappointed by this novel. "Cathedral" lacks authority in many small, but telling, ways.

"Londonderry," for example, does not exist in the vocabulary of Irish nationalists. They strip the name of its British weight and call the place simply, but adamantly, "Derry." Yet De Mille has his hard-boiled Irish rebels thinking and talking about "Londonderry." The songs De Mille has his Irish patriots sing are also out of character. With two passing exceptions, their repertoire consists of a half-dozen or so of the most over-heard cliches in Irish music, including "Danny Boy," "Molly Malone," "I'll Take You Home Kathleen," "How Are Things in Glocca Morra?" and "My Wild Irish Rose" -- a much more appropriate repertoire for the one-day-a-year Irish who turn out on St. Patrick's Day than for a group of Irish rebels. They should be singing "The Town I Loved So Well." And the only Irish anyone in this book utters is "Erin go bragh." One gets the impression De Mille worked much harder grasping New York City's police procedures than in trying to understand his central characters.

One of his central characters (of sorts) is St. Patrick's Cathedral. The ins and outs of St. Patrick's are delineated during the course of the story, yet nowhere does De Mille give us a history of the cathedral, or tell us what treasures it houses, or even (on a more mundane level) how many dollars its destruction would cost -- sins of omission, rather than commission, but still disappointing.

Although the archvillain in the novel turns out to be a very nasty British intelligence officer, and Lt. Burke, the Irish American cop, emerges as the hero, one senses that their roles are an attempt to cancel out the negative depiction of the Irish guerrillas as monsters "spawned in hell" who easily manipulate all those Irish Americans, those sentimental creatures with "beery tears rolling down red faces."

If the writing were brilliant, "Cathedral" would rise above all these flaws. There is certainly plenty of action and suspense built into the plot, but the prose is not distinguished. For example: "The draft was eerie also because he couldn't quite tell from which direction it came. It seemed to come from some hidden respiratory organ belonging to the Cathedral itself -- in a way, the secret breath of St. Patrick's -- or St. Patrick himself." This passage is one of the many that makes you want to say, "Oh, c'mon." And it brings to mind a comment Burke offers after crazy John Hickey's television appearance: "Really hokey, but . . . it gets them every time." Lt. Burke's comment could pass easily for Nelson De Mille's motto.