Shortly before midnight on May 10, a small group of carolers will gather in front of a five-story brick mansion facing the East River on New York's posh Beekman Place.

When the clock strikes 12, this impromptu glee club, led by composer and cabaret performer John Wallowitch, will break into "Happy Birthday," followed by a chorus of "Always."They will be celebrating at the doorstep of the man who wrote that haunting love song. Irving Berlin, America's best-known - yet most private - songwriter turns 100 on May 11, 1988.

Just mention the melodies Berlin wrote, and you'll start to sing: "White Christmas," "Easter Parade," "God Bless America," "There's No Business Like Show Business," "Alexander's Ragtime Band," "A Pretty Girl Is Like a Melody," "Puttin' on the Ritz," "Blue Skies," "How Deep is the Ocean?," "Cheek to Cheek," to name a few.

Wallowitch's Berlin salute, an annual celebration he began more than 20 years ago and which he usually holds at 6 p.m. on Christmas Eve, may be the most personal tribute to Berlin this year.

"They have been devastatingly emotional experiences," says Wallowitch, an unabashed Berlin fan who lives down the street and around the corner from the composer.

But the only festivity officially recognized by the songwriter will be a Carnegie Hall salute on May 11 held in connection with ASCAP, the American Society of Composers, Authors and Publishers. Berlin is a charter member of the organization - founded in 1914 - that licenses music for public performance.

The collection of diverse performers expected to appear at the gala includes Frank Sinatra, Willie Nelson, Jerome Robbins, Walter Cronkite, Isaac Stern, Ray Charles and Garrison Keillor. It will be taped by CBS for a television special later in the month.

Don't expect an appearance by the master. Berlin is passionate about his privacy. Except for short walks around Beekman Place when the weather is warm, Berlin never is seen in public. On his infrequent strolls through the tranquil, tree-lined neighborhood, he usually is accompanied by a young woman or a doorman from a nearby building.

Call the Irving Berlin Music Corp. and ask for an interview with the boss. You'll get Hilda Schneider, Berlin's longtime secretary. She is polite and friendly but adamant in declining interviews for her employer.

"I'm sorry I can't be more encouraging," the woman says. "If we do one, we'd have to do them all. It's all in the record. You can look it up."

And the record is pretty impressive: some 1,500 songs that span the worlds of theater, movies and popular music. No other composer can match that record.

For the stage, Berlin wrote "Annie Get Your Gun" and more than a dozen other Broadway shows. His movies include several Fred Astaire-Ginger Rogers classics such as "Top Hat" and "Follow the Fleet." His pop hits, those songs not written for any particular stage show or film, include such standards as "Remember" and "All Alone."

What makes his music so great?

"His songs have the sophistication of simplicity," says Morton Gould, president of ASCAP. "You know precisely what he's saying and singing. He's been able to distill everything down to its essence.

"He also expresses universal truths," Gould adds. "If you look at his song titles and listen to his music, he expresses emotions that human beings would like to experience. Most people want blue skies. They want the equivalent of white Christmases or the whole idea of what `Easter Parade' evokes. And most of us don't want to be all alone by the telephone. He says it directly and you feel it."

Ironically, the telephone is how Berlin communicates with the outside world today. When he wants to talk, he calls. The songwriter has a small circle of telephone friends, people he dials when he has something on his mind. Gould is one such listener. "He will just call," Gould says. "His liaison will say `Mr. Berlin would like to talk to you.' When that happens, you drop everything and pick up the phone. It's a wonderful thing. He has a keen mind, and he's right on top of everything."

Another member of this select telephone circle is Stanley Green, a musical comedy historian and author of several books about the Broadway musical. Green last spoke to Berlin when Fred Astaire died last year. Berlin called Green to express his regret about the passing of a man he admired very much.

Most of Berlin's contemporaries are dead. He's the last of the great musical comedy composers, a pantheon that once included George Gershwin, Jerome Kern, Cole Porter and Richard Rodgers. Of his theater contemporaries, only director George Abbott, who turns 101 in June, survives. Berlin, born Israel Baline in Russia, came to the United States at the age of 2. His family settled on New York's Lower East Side where, as a young boy, Berlin got jobs singing on the Bowery. He worked as song plugger and a singing waiter at a Chinatown cafe. It was there that he learned to play the piano, laboriously picking out tunes with one finger. This self-taught musician writes the same way.

`It's an incredible catalog this man has achieved with no formal education as well as no musical education," Green says. "And to be able to write music AND lyrics."

For all his success, Berlin is a loner, a man very much aware of his humble origins. His first wife, Dorothy, died from typhoid fever less than a year after they were married.

In 1926, Berlin married his second wife, Ellin Mackay, the daughter of a wealthy and socially prominent businessman, Clarence Mackay, the president of a commercial telegraph company. She eloped with Berlin over her Catholic father's objections, and the marriage was front-page news all over the country.

In the more than 60 years they have been married, Berlin has kept his wife and his three daughters out of the public eye.

"I think a reason he is so very private is that he is wildly in love with his wife," Wallowitch says. "I think it is one of the greatest romantic love affairs in history." Berlin is the only major songwriter who never had a film biography like Cole Porter's "Night and Day," George Gershwin's "Rhapsody in Blue," Sigmund Romberg's "Deep in My Heart" or Rodgers and Hart's "Words and Music." Coupled with his desire for privacy was Berlin's shrewd business instincts. In the early 1920s, he and two partners built the Music Box Theater to showcase his musical revues. He and the Shubert Organization still own it.

His music publishing company tightly controls his songs. ASCAP won't reveal the amount of money these melodies have generated, but the man is believed to be a millionaire many times over. Berlin gave the royalties from "God Bless America" to the Boy Scouts and Girl Scouts of America. So far, they have earned some $700,000 from that one song.

Berlin withdrew even more from the public eye in 1962 after his last Broadway musical, "Mr. President," flopped. The show, starring Robert Ryan and Nanette Fabray, was a wildly anticipated musical, generating a record box-office advance but tepid reviews. It had a six-month run.

Still he came back. Four years later, he wrote a new song, "An Old- Fashioned Wedding" for the Lincoln Center revival of "Annie Get Your Gun" that starred Ethel Merman. Twenty years after the original production, she stopped the show again with Berlin's new song.

In all his years of serenading Berlin, Wallowitch finally was invited into the Berlin home nearly five Christmas Eves ago after a chorus of "White Christmas." On an impulse, he rang the doorbell. A maid answered the door and he and his fellow singers were taken into the kitchen.

There they were greeted by Berlin himself, wearing a bathrobe and slippers. The composer hugged the men, kissed the women and told them, "That's the nicest Christmas present I've ever had."

Every day, Wallowitch walks his dog Winnie around Beekman Place and often wonders what is going on inside the big house now. "I keep holding on to the idea that the man is not allowing himself to be bored and is still writing," Wallowitch says. "God, I hope it's true."