A young man bearing an Irish Republican Army flag and screaming "Murderers!" rushed at the Duchess of York, the former Sarah Ferguson, Thursday night in front of her midtown Manhattan hotel.

The duchess, who was returning from a Broadway benefit performance of "The Phantom of the Opera," apparently was unhurt in the attempted assault in front of the Waldorf-Astoria Hotel at about 11 p.m. A British Embassy spokesman said the duchess was "perfectly relaxed about it."

The suspect, identified as Michael Shanley, 22, of Manhattan, was ordered held without bail yesterday at the federal Metropolitan Correctional Center in Manhattan. He is charged with assault and attempt to assault with a deadly weapon a "foreign official, official guest and internationally protected person ... the duchess of York," and also with assaulting State Department security officers. If convicted, he faces up to 20 years in prison and a $500,000 fine.

U.S. Magistrate Leonard Bernikow described the deadly weapon as a "solid wooden flagpole with a metal base." He ordered Shanley held until a detention hearing can be held Tuesday.

Shanley was described by his lawyer, Stephen Somerstein, as an unemployed waiter who attended the College for Human Services but was not registered for the current semester.

A British Embassy spokesman, who was in New York with the duchess, said New York police "do a very fine job ... They anticipate all sorts of things that, thank goodness, do not happen." Reagan & the Bronco Shorts

Rep. Pat Schroeder (D-Colo.) received an urgent call from President Reagan yesterday regarding an unsettled matter concerning football.

A couple of weeks ago, Reagan, when in Cleveland for the Browns-Broncos game, was photographed holding up a Browns T-shirt. The photograph, later splashed on the front pages of Denver's papers, sent Broncos fans into a tizzy.

In response to the photo, Schroeder sent a letter and a pair of Bronco shorts to the president, expressing her disappointment in his display of favoritism.

She also added, "Real men wear Bronco shorts."

When Reagan received the package, he called Schroeder to say that he had been given the T-shirt as a gift and that he had not meant to offend Bronco fans. Reagan said he was not taking sides.

Schroeder said yesterday that she hopes the president will wear the shorts during his State of the Union address ...

Fred Fillah, president of Shirt Explosion in Hyattsville, has $500,000 worth of orders for T-shirts paying tribute to Redskins quarterback Doug Williams. The shirts say "Doug Williams -- A Touch of Class." Mr. Redford Comes to Washington

Robert Redford prompted a few shrieks Thursday from a normally sedate Senate staff when he stopped by the Capitol for a visit with Sen. Bill Bradley. Word of the star's visit, though unpublicized by Bradley's office, quickly hit the Capitol Hill grapevine and drew about 50 onlookers to the hall outside the New Jersey Democrat's office.

Bradley and Redford, who met when the senator was a professional basketball player, talked privately for about 30 minutes, said Bradley spokesman Nick Donatiello. "They're old friends. He just dropped by to say hello and have a cup of coffee," Donatiello said. It was unclear just what brought Redford to Washington. Cybill Shepherd's New Star

Calling it "a dream come true," actress Cybill Shepherd on Thursday became the latest celebrity to be honored on Hollywood's Walk of Fame.

In a ceremony on Hollywood Boulevard, Shepherd, the star of ABC's "Moonlighting" and a recent mother of twins, said, "People can now walk all over me like they've done for years and I won't mind a bit." The television star's husband Bruce Oppenheim and 300 fans watched as the 37-year-old Shepherd lay down on the sidewalk next to her star to pose for pictures.