The news of a successful summit in Northern Ireland was swamped in the flood of blather from the other summit in Geneva.

It's too bad for several reasons. First, British Prime Minister Margaret Thatcher and Irish Prime Minister Garrett FitzGerald brought out of the Ulster Castle of Hillsborough an agreement promising modest but significant progress in a 60-year-old problem that has been a synonym for stalemate. Second, Hillsborough was a kind of model summit, which illustrates the virtues of careful preparation, super-discretion and a clearly agreed-upon agenda.

Eighteen months of intense work went into it: In addition to two full summits between the principals, there were four informal meetings at European Common Market gatherings, six ministerial meetings and 35 at lower levels. For a full year, the all-Ireland Forum took volumes of testimony from all plaintiffs, north and south.

Thatcher and FitzGerald have known each other for 10 years. They approach the problem of Northern Ireland, as FitzGerald said at their joint news conference, "from wholly different historical perspectives and title-deeds."

But they always got along, maybe because both have been teachers and share intellectual curiosity. Besides, they were often thrown together at European meetings; and FitzGerald, a most obliging and civilized man, sometimes served as her interpreter, since Thatcher knows only English and he speaks most of the languages of Europe.

"The chemistry was good, even when the meetings didn't turn out so well," said Sean Donlon, former Irish ambassador to the United States and secretary of the Department of Foreign Affairs. He was referring to the dark moment a year ago when, after their first summit, Thatcher brusquely dismissed each possibility for better relations as "not on."

Thatcher and FitzGerald had to put aside a great deal more than Reagan and Gorbachev. Despite a history of almost hysterical hostility between the United States and the Soviet Union, the Soviets and Americans have never gone to war. To talk rationally about Ireland and England, each representative must put aside the memory of rivers of blood.

But Thatcher and FitzGerald managed it. Her personal introduction to the Irish problem was brutal -- the murder of her friend Airey Neave, whom she had chosen as her minister of Northern Ireland. In October 1984, she escaped an IRA bomb in a Brighton hotel. She showed great courage and magnanimity in going to Ireland for the signing, and agreeing to have the Union Jack hauled down from the castle.

Thatcher's motivation is similar to that ascribed to Reagan, the desire to go down in history as a peacemaker. A solution to the Irish problem has evaded all her predecessors. It is an achievement she covets. FitzGerald was born wanting reconciliation: His mother was a northern Protestant, his father a southern Catholic.

Their agenda was neither crowded nor in dispute. They had no spurious photo opportunities, no spouses' teas or big-wig news conferences. They wanted to give the Republic a say in Ulster's affairs, to improve relations between the Ulster security forces and the nothern Catholic minority. They didn't talk about exchanging Erse scholars. They established an Anglo-Irish Conference to ensure cooperation on political, security and legal matters. They promised there would be no change in Northern Ireland's status without majority consent. Thatcher and FitzGerald rehearsed their post-summit statements so that no unnecessary offense would be given or taken.

The reaction was as expected. Ian Paisley, the militant Protestant demagogue, howled betrayal, and Northern Ireland's parliament denounced the accord. But Paisley promised to keep resistance within constitutional limits. He proposed that Irish members of Britain's parliament resign, forcing a referendum of sorts; if reelected, they would resign again. Such disruption is regarded as preferable to taking guns into the streets or calling a general strike.

The leader of FitzGerald's opposition, Charles Haughey, complained that the agreement "recognizes the legitimacy of unionism." U.S. support was instantly assured. President Reagan and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) jointly promised aid to a peaceful Ulster.

It was all in admirable contrast to the mindless hyper-activism in Geneva, where two total, bristling strangers are meeting amid a mob of reporters who are playing the super-power encounter like the Olympics -- "us against them."