John Paul II, a pope who breaks precedent in almost everything he does, adds two more historic firsts tomorrow as he leaves for Ireland and America. He will become the first pope ever to set foot on Irish soil, and then the first to tour the United States.

For John Paul, his trip marks the end of an unusually active first year as spiritual leader of 700 million Roman Catholics around the world. He already has established himself as a vigorous pope who often defies tradition and who intends to make his presence felt on what he calls personal pilgrimages. His Irish and American trips, coming after earlier emotional ones witnessed by millions in Mexico and his native Poland, follow that pattern.

"I express my indestructible faith that my visit to Ireland will serve the great cause of peace and conciliation so much desired by the entire Irish nation," he said last Sunday while asking a crowd of 50,000 here to pray for the success of his journey.

He also asked them to pray "that I might accomplish my service in the land of Washington."

The pope, the first non-Italian to sit on the throne of St. Peter in Rome in nearly five centuries, and at 59 the youngest in more than 130 years, will end his latest journey after becoming the first pontiff to be received by a president in the White House. He returns to Rome after celebrating a mass in Washington attracting 500,000 to a million people. Millions more are expected to turn out to see him as he moves through a hectic three-day schedule in Ireland and then completes a six-city visit in the United States.

His trip begins early Saturday morning when he leaves Rome's Leonardo da Vinci airport for Dublin. After a brief welcoming ceremony, he goes by helicopter to the residence of the papal nuncio, and then he celebrates an open-air mass in Dublin's Phoenix Park. That crowd is expected to number up to a million people -- just under a third of the entire Irish Republic's population.

The pope's visit to volatile Ireland comes at a time of bitterness and bloodshed between Protestants in Northern Ireland and the overwhelmingly Catholic population in the south. Last month's assassination in Ireland of Lord Mountbatten, the World War II hero and member of the British royal family, has heightened concern about the pope's security.

In recent days, there have been published reports that members of a West German terrorist group have traveled to Ireland in advance of the pope's arrival, supposedly to meet secretly with the Irish National Liberation Army. The INLA is described as a Marxist splinter group of the Irish Republican Army, and it is said to consider the pope an enemy because he has denounced political violence.

The specter of violence in Ireland creates inevitable uncertainty about preparations for elaborate national celebrations during the first papal visit. Despite the pope's stated hopes for Irish peace and conciliation, the recent public comments of both warring sides in the long civil strife have been hard and unyielding.

In Belfast a leader of the IRA, the group that claimed credit for Mountbatten's murder, was quoted as saying, "There will be no ceasefire during the pope's visit." Pope John Paul had sent Queen Elizabeth a special message of condolence at the time of Mountbatten's death, condemning the assassination as "a tragic murder" and "an insult to human dignity."

Vatican spokesmen say security arrangements made by the Irish are "very tight" and that "great restrictions" have been placed on movement during the papal visit. As only one indication of the obvious seriousness with which the Irish authorities are approaching their task, two Irish security agents have been flown over here to supervise personally the handing out of credentials to those accompanying the pope on his plane.

Security is only one of the concerns' confronting the Vatican on this trip. Until Pope Paul's reign, the Vatican had no experience in planning for the complicated foreign journeys that are both visit from a chief of state -- Vatican City is the world's smallest city-state -- and visit to far-away parishes of the Roman Catholic Church.

Preparation of the pope's prepared remarks and their translation into several languages, as well as the often-delicate protocol requirements, are only part of the problems. The press arrangements, -- the political considerations, the cities chosen to visit -- all place a strain on a Vatican historically remote, closely-knit, and secretive, and in many respects ill-prepared to deal with such massive public excursions.

The advent of John Paul's reign has placed even greater burdens on the already-taxed Vatican.

This new pope, the former Cardinal Karol Wojtyla of Krakow, Poland, clearly intends to carry his papal message personally around the world. His freshness of style, his athletic appearance, his penchant for dispensing with tradition, and his extensive travels have attracted intense public interest in him.

And this trip, his third major foreign one in a year, has become what one priest helping plan the arrangements from here calls "the most complicated of all papal journeys." In little more than a week, the pope will be visiting three separate jurisdictions -- Ireland, the United States, and the United Nations. Each requires special security provisions and credentials, and each poses different challenges in how the pope addresses the specific general audiences.

"This one is more complicated than Poland," the same priest says, referring to the pope's homecoming last spring. In Poland, he adds, at least a certain unity existed -- a cohesive church and an oppressive state.

That will not be the case in this most venturesome papal journey yet.

In Ireland, the pope's schedule calls for a number of trips and special ceremonies. Saturday afternoon he goes by helicopter 30 miles north from Dublin to Drogheda to visit a shrine.Later in the evening he meets bishops and leaders of other churches. On Sunday, he travels to Galway to celebrate a mass and also conducts one at the Knock Shrine. His last day in Ireland, Monday, includes several more helicopter trips -- to Maynooth, to Limerick, and finally to Shannon airport for his departure to the United States.

That afternoon he arrives in Boston and continues at a hectic pace with visits to New York and the United Nations on Tuesday, to Philadelphia on Wednesday; to Des Moines and Chicago on Friday; and then on to Washington Saturday. That Sunday afternoon he celebrates the massive outdoor mass on the mall stretching between the Capitol and the Washington monument.