For a visiting Irish politician, John Hume, the Tip O'Neill 50th anniversary-in-politics banquet was awesome. It wasn't just the mammoth crowd, some 2,400; it was the sight of the president of the United States and the speaker of the House, the one an ideological conservative, the other a partisan liberal, standing together, swapping cracks and compliments on the eve of a bitter showdown on an issue each one feels he has to win.

"It's the civility," said Hume, in whose native Ulster most disputes are settled at gunpoint, "the acceptance of diversity -- it's something we're just trying to learn."

Nobody wanted to be in the jammed ballroom of the Washington Hilton more than the president. He made it plain that he wouldn't have missed the occasion for the world. He was obviously determined to do well before an Irish crowd that belonged to somebody else. He was at the top of his form. He delivered funny, flawlessly timed remarks about his rivalry with O'Neill, made jokes about his age, his hearing, his memory. The crowd was delighted and disarmed.

Former president Gerald R. Ford had also taken much trouble with his text, which was in the self-deprecating vein of the evening. Following Bob Hope, another of O'Neill's golfing partners, he said he hadn't been in such a fix since "just before I fell down the ramp in Salzburg, Austria."

No speaker, of course, mentioned the issue on which O'Neill and Reagan have most lately locked horns, contra aid. Instead, there were many tactful references to the one cause that unites them, aid to Northern Ireland. "We always hope to promote peace in Northern Ireland," said O'Neill.

Some could have done with a little less talk about golf, O'Neill's avocation, and a lot more about politics, his vocation. The speaker, after all, is the leader of the Democratic Party, its national voice and its bulwark against rampant Reaganism.

But the affair was totally satisfying to the speaker. He had his wife of 45 years, Millie, and his five children before him. He had raised $2 million for his alma mater, Boston College, he had received honorary citizenship of Ireland and a road sign pointing to his ancestral town of Buncrana, in County Donegal.

He had heard himself praised by a number of people whom he regards as "beautiful individuals." A Boston Irishman said, "It was like being at your own wake." The event was pure O'Neill -- warm, massive and liberally laced with laughter and reminiscence.

"I, Tip O'Neill, am grateful to all of you," he said in his closing remarks, which like all his utterances were comfortable, colloquial and informed with his ancient New Deal faith.

The morning after, the two old Irishmen were back in the trenches, going at each other hammer and tongs.

At his noon briefing, the speaker said of his rival, "He's a charmer, isn't he?" -- and then predicted he would beat him in today's vote. The president, meanwhile, was luring some of O'Neill's errant charges to the White House to "negotiate" a compromise to confound the liberals.

In the corridor outside the chamber, the speaker's lobby, some of the younger liberal Democrats, who have in their time railed at O'Neill's leadership -- too partisan, not substantive -- were paying their own tribute and mourning his departure at the end of the session.

"The Democratic Party will lose a huge chunk of personality when he goes. That overwhelming kindness, the comfort he gave to so many poor people. He looked big enough to protect them," said Rep. George Miller (D-Calif.).

There was a time when Democrats bewailed the fate that had given them a caricature of an old, cigar-chomping pol to go up against the sleek performer in the White House. They wanted him to diet; they wanted him to collect nuances on issues, to go on TV talk shows or at least master the art of the 30-second sound bite for the evening news.

Republicans chortled that they had a blunt instrument for realignment, and brash John LeBoutillier of New York in 1982 ran his reelection campaign on the O'Neill issue -- "big, fat and out of control -- just like the federal budget." LeBoutillier lost, and O'Neill began to go up in the polls. The girth has been rethought, a metaphor for shelter against the Reagan storm.

O'Neill's plain speech -- he talked about Nicaragua in terms of Vietnam and what his old Aunt Eunice of the Maryknoll Order told him about what was really going on in Managua -- began to be seen as bracing to the waverers.

The massive banquet was a real celebration -- of a politician who made it and remained himself, who kept in mind his father's counsel, "to always remember from whence you came."