It takes a 4-pound briefing book to chart Reagan's travels

It weighs more than four pounds. It is chock full of data about President Reagan's trip to Ireland, France, and London, and it is just one of the items the horde of journalists traveling with the White House party laboriously tote from one stop to another to cover the President. All in all, it must add at least 1,000 pounds to the airfreight.

''It'' is the two-inch-thick, hard-cover, spiral press briefing book, put out by the State Department and emblazoned with the presidential seal, which the White House has handed out to every member of the press group. And, in a small way, it illustrates the massive organization that goes into a presidential roadshow, a show that on this occasion includes an entourage of 600, including government officials, security personnel, and some 250 journalists.

Never far from the reporter's side, the briefing ''bible'' opens with a ''Dear Members of the Press'' letter from Ronald Reagan.

''Our journey to Europe is one of special importance,'' the President tells the press. ''Your role, as always, is a central one as you bring to citizens across the globe the story of how we meet the challenges we face. Nancy joins me in welcoming you on this important trip.''

Then follows neatly compartmentalized information for every leg and stage of the 10-day trip: the daily schedule of events; a list of members of the traveling press; flight times and distances; biographies of the participants in the London summit meeting; notes on customs; currency conversions; texts of background briefings by ''senior administration officials''; profiles of the countries being visited; trade tables; speeches by key summit participants; maps of countries, towns, and the Normandy landings; fact sheets on US trade policy and previous economic summits; charts on gross national products, deficits, consumer price increases; explanations of abbreviations like COCOM, MBFR, OECD.

Even advice on how to pronounce and stress leaders' names: ''meetairRAHN,'' ''trooDOH,'' ''GHENsher,'' ''nahkahsohneh.''

''It's like a security blanket,'' one veteran White House reporter comments. ''It's not only the factual information it contains. But if you should be at a loss for words, you can always fall back on something in the book.''

To be sure, much of the background material is peripheral. Journalists would be hard pressed to make use of the fact that Nancy Reagan was ''named one of the 10 most admired women in the world by readers of Good Housekeeping magazine''; that Desmond's cave at Ballyporeen ''is famous because the Earl of Desmond sought refuge there only to be betrayed in 1601''; or that Margaret Thatcher likes listening to Bach, Beethoven, and Chopin.

But even a casual flip through the book turns up items of historical, political, and economic interest. For example:

* The President's name could just as well have turned out to be O'Regan. His great-great-grandfather was Thomas Regan or possibly O'Regan, from the Irish county of Tipperary.One of Thomas's six children, Michael Reagan/Regan/O'Regan, left for England at the time of the potato famine and there spelled his name Reagan. In 1858, he and his family emigrated to America.

According to the White House genealogical rundown: ''The spelling Reagan is generally unknown in modern Ireland, and such a spelling appears in no Irish telephone directory. The local version is Regan (or O'Regan, the 'O' prefix often being dropped) and is pronounced 'Reegan.' ''

* The US Army is not beyond a little historical revisionism. A news release about the D-Day landing cites Dwight Eisenhower, commander of the Allied forces, saying in a radio broadcast to Europe on June 6, 1944: ''The landing is part of the concerted United Nations plan for the liberation of Europe. . . . I call upon all who love freedom to stand with us now.'' Deleted is the phrase ''in cooperation with our great Russian allies.'' (United Press International, in a recent report, quotes the author of the news release as saying the deletion was made to avoid confusion. ''It would have sounded as if the Russians had taken part in Normandy.'')

* Japan, it seems, is not always at the top of the economic indicator lists. Of the 21 countries of the Organization for Economic Cooperation and Development , Turkey - yes, Turkey - had the highest rate of economic growth over the decade 1973-83. Japan was second and Norway third. Taking just the past five years, however, Japan returns to its preeminent status; Turkey drops to a still respectable sixth place.

* Trade with the poorer nations of the world is of increasing importance to the United States. It now accounts for about 40 percent of all US exports.

* The United States and world energy markets are in better shape today than they were at the time of the great Arab oil embargo. The US imported only 28 percent of its oil last year, compared with 35 percent in 1973. Moreover, it imported only 60 percent as much oil from the OPEC countries as a decade ago.

As the presidential trip progresses, the briefing book will grow heavier and heavier as reams of on-the-road briefing transcripts, pool reports, and other material are added to the load. By journey's end, many of the weary reporters will be looking for an extra tote so the book can be stored by White House transportation personnel for return to Washington.

Not to worry. One of the perquisites of a presidential trip is a flight bag handed out gratis by the airline. The briefing book fits nicely.