To people in Ireland, St. Patrick's Day isn't that big an event. They have nothing to compare with the festivities in cities like New York, Philadelphia, Boston and Chicago, where even the Chicago River is colored green. This year, it can easily be argued that the most important St. Patrick's Day celebration takes place this evening at the Washington Hilton, where the nation's most famous Irish-American is being honored for his 50 years in elected politics. It's true that President Reagan is of Irish descent, but when it comes to being Irish, House Speaker Tip O'Neill is the first name that comes to mind.

Tonight's celebration is so important that even Irish Prime Minister Garret FitzGerald is flying in on the Concorde from Ireland for the $1,000-a-plate dinner. The lineup of speakers includes Reagan, former president Gerald Ford, Sen. Edward Kennedy, comedian Bob Hope and FitzGerald. The money raised, expected to total more than $2 million, is designated for a scholarship fund at the speaker's alma mater, Boston College. It was shortly after graduation in 1936 that O'Neill, at the age of 24, was first elected to the Massachusetts House of Representatives. Then in 1952 he won the seat John F. Kennedy vacated when he was elected to the Senate.

The hotel probably won't be serving green beer, but there will be so many politicians and lobbyists working the room that O'Neill, the master at that game, should feel he's back home working the district. The music will be the songs of Robert White, an Irish tenor. And with very little persuasion, the speaker will undoubtedly break into song himself. Listen for the opening chorus of "If You're Irish, Come into the Parlor . . ." O'Neill is no threat to White's singing career, but he's enthusiastic.

And last night, there was another important gathering of the Irish. At a reception at the Powers Court Restaurant in the Phoenix Park Hotel, the Committee for a New Ireland feted such sons of the old sod as Prime Minister FitzGerald, Kennedy, Irish Ambassador to the U.S. Paddy MacKernan and U.S. Ambassador to Ireland Margaret Heckler. The evening was serious, but they did serve corned beef and cabbage.

End Notes

A veteran royal watcher says it's wedding bells for Prince Andrew and his girlfriend Sarah Ferguson. In today's Daily Mail, columnist Nigel Dempster declares, "Prince Andrew and Sarah Ferguson are engaged." Buckingham Palace refused to comment.Dempster's column adds: "It is believed that the queen will make an official statement before Sarah's father, Major Ron Ferguson, leaves for Australia on Wednesday." Speculation about a wedding began last month when Ferguson, 26, visited the ship of the prince, a navy officer, and then went skiing in Switzerland with Prince Charles and Princess Diana . . .

What with St. Patrick's Day and all, the Irish must be feeling especially kindhearted. The Bhagwan Shree Rajneesh, who is having a difficult time finding a place to set up shop again, has been given an entry permit into Ireland. Rajneesh, the Rolls-Royce-collecting Indian guru who was kicked out of the United States and Greece and barred from Britain, is now in Limerick with eight of his disciples. One would think the Irish have enough problems. His permit is good for three months. The guru reportedly said he plans to fly to Antigua, but the government of that Caribbean island has said it doesn't want him either . . .

This year's Helen Hayes Awards celebration on April 28 at the National Theatre promises to truly be a night of theater stars. Helen Hayes will be hosting the awards, and Anne Jackson and Eli Wallach, who have worked on numerous Washington stages, will be masters of ceremonies. The guest award presenters scheduled to appear include Jason Robards, Jose' Quintero, Lena Horne, Richard Kiley, Eva Marie Saint, Julie Harris, Vincent Price, Charles (Honi) Coles, Rosemary Harris, Robert Prosky, Ming Cho Lee, Art Buchwald and Eileen Brennan. Some of the musical numbers will include selections from "La Cage aux Folles," "Forbidden Broadway," "Barbara Cook -- A Broadway Evening," "A Chorus Line" and "Banjo Dancing" . . .

Victoria Sellers, who can be seen in photographic detail in the April issue of Playboy, has been released on $100,000 bond after surrendering to authorities to face charges she was part of an international cocaine ring. The 20-year-old daughter of the late actor Peter Sellers and Swedish actress Britt Ekland, is reportedly talking to drug enforcement officials about cocaine use among her peers in Hollywood. Sellers is charged with being part of a drug ring that sold $1 million worth of cocaine a week in Los Angeles . . .

Actor James Cagney was resting comfortably this weekend after being treated for circulatory problems in his legs in Lenox Hill Hospital in New York City. The 86-year-old movie legend is expected to be released sometime this week . . .