Pope John Paul II has canceled a plan to visit Northern Ireland because of the recent violence there, the Vatican announced yesterday.

A tentative decision to visit Ulster during the pontiff's upcoming 10-day tour of Ireland and the United States was reversed "with great regret," a Vatican spokesman said, "due to the brutal murders of recent days." On Monday, British was hero Lord Mountbatten, members of his family and 18 British soliders were killed by Irish Republican terrorist bombs.

The Vatican for the first time also officially confirmed previous reports on what cities the Pope will visit during the seven days he will be in the United States in October.

The pope will arrive in Boston from Ireland early on the afternoon of Oct. 1, according to church officials here. He will fly to New York and address the United Nations Oct. 2, visit Philadelphia Oct. 3, and Des Moines, for a few hours, on Oct. 4. He will spend the nights of Oct. 4 and Oct. 5 in Chicago and then arrive in Washington late in the morning of Oct. 6, officials said.

John Paul II will be the first pope to visit the White House and is expected to celebrate a mass for as many as 1 million people on the Mall while he is here.

Father Maurice Fox, spokesman for the Washington archdiocese, said yesterday that some details remain to be worked out, but the basic plan for the pontiff's stay here calls for him to land at Andrews Air Force Base, then take a helicopter to a site near the Lincoln Memorial reflecting pool.

A motorcade will take him to St. Matthew's Cathedral where he will meet with as many as 1,200 priests.At about 1:30 p.m., the pontiff will be driven to the White House to meet for threee hours with the president and prominent political leaders from the major branches of government, Fox said. As many as 5,000 people are expected to gather on the South Lawn for the occasion.

From the White House, the pope will go to the Organization of American States for about an hour to meet with members of the diplomatic corps, then to the residence of the apostolic delegate across from the Naval Observatory on Massachusetts Avenue, N.W., where he will spend the night.

At 8:30 a.m. on Sunday, Oct. 7, the pope will go to the National Shrine of the Immaculate Conception in Northeast Washington to address a group of nuns.

He then will visit nearby Catholic University to speak to a convocation of the nation's leading Catholic academicians.

The papacy has a unique relationship to CU, the only university in this country originally chartered by a pope -- Leo XIII, in 1837.

John Paul II also will address a gathering of ecumenical leaders from various protestant denominations at the university's Hartke Auditorium before returning to the apostolic delegate's residence at 12:30 p.m.

A motorcade will take the pope to the Mall at 3 p.m. Oct. 7, where he will celebrate mass for crowds expected to stretch for two miles from the Capitol to the Lincoln Memorial.

Local police are girding for the task of handling the tremendous numbers, and will cancel all leaves throughout the October weekend, District and federal officials said yesterday.

The pope will address the crowd from a three-tiered stage set between the Smithsonian castle and the Museum of Natural History, using the services of as many as 1,500 eucharistic ministers to deliver communion to the multitudes. Pope Paul II will leave Washington for Rome the evening of Oct. 7.

Large outdoor masses similar to the one on the Mall will be celebrated in each of the other cities the pope visits. All are expecting crowds upwards of 200,000 on this first extensive U.S. tour by a head of the Roman Catholic Church.

Pope Paul VI visited the United States in 1964, but stayed only a few hours to address the United Nations and hold a mass in New York's Yankee Stadium.

Though there have been reports that the pope may return to this country in 1981, religious leaders in areas he will not reach this time around said yesterday that they were disappointed in John Paul's decision not to visit the West and Southwest.

"There's a lot of America west of Des Moines," observed Jim Jennings, communication officer of the Phoenix, Ariz., diocese.

Church officials said the far West was not included on this trip in order that the pope can spend more time meeting people than traveling.

The cities to be visited by the pope in Ireland between Sept. 29 and Oct 1, also announced yesterday, will include Dublin, Drogheda, Galway, Knock, Maynooth and Limerick.