A full-dress rehearsal had followed a high-level planning session yesterday morning, and by noon the show went on -- a technicolor (St. Patrick's Day green) version of the weekly Monday issues lunch.

President Reagan said later that he'd never seen anything like it, and small wonder. Leprechauns, Irish jigs, bagpipes and kilts aren't your standard White House fare. Which is maybe just as well.

"Guess who's coming to lunch," quipped Reagan to photographers as he sat down at the table in the Cabinet Room. Unaware of the surprise attractions to come, he was referring to himself.

"We told the president there'd been a lot of requests for a photo opportunity," Deputy Press Secretary Larry Speakes explained later.

What an opportunity. While the cameras ground away across the table from him, Reagan turned his head to watch Eric Rice-Johnston play his bagpipes, Linda Rice-Johnston dance a jig and Daniel Sugrue sing "Oh, Ronnie boy, oh, Ronnie boy, I love you so," to the tune of "Danny Boy."

The finale was professional clown Alfredo Comignani, rising to his full 4-foot-2-inch height standing on the chair next to Reagan to blow up a green balloon that was purported to be a shamrock, though not everyone recognized it as such.

Reagan was clearly in his element. "Any time people hear an Irish song sung they wish they were Irish, too," he told his staff.

After that everybody got down to the real issues of the day: corned beef and cabbage, boiled potatoes and carrots and green ice cream.

The corned beef was courtesy of chief of staff Donald Regan, who, in turn, got it courtesy of New York lawyer Peter Flanigan, a former Nixon aide. "He sends it every year," Speakes said.

Those sweet nothings Canada's Prime Minister Brian Mulroney and his wife Mila will be eating at the lunch George Shultz is giving for them today will be chocolate tulips filled with raspberries and topped with cream.

In the April edition of House & Garden magazine, Chief of Protocol Selwa Roosevelt tells about another chocolate dessert, one created for the king and queen of Nepal, that never made it to the table. Made of white and dark chocolate studded with red raspberries, the dessert resembled a crown, Roosevelt said yesterday. She thought it sensational until the Nepal desk officer warned her that it would never do to have the king plunging a knife into a crown.

Since then Shultz's menu planners haven't taken any chances. "We were afraid to do the maple leaf," Roosevelt said of Canada's national symbol, "so we went to the tulip."

The Reagans' White House dinner tonight for the Mulroneys will be the first for a Canadian prime minister since Pierre and Margaret Trudeau paid Jimmy and Rosalynn Carter a visit in 1977. That was the night Margaret showed up in a short white wool dress, shocking the fashion press. That was also the night Amy Carter read two books during dinner, shocking everybody else.

On tonight's guest list are three native Canadians -- actress Kate Nelligan, actor Christopher Plummer and architect Arthur Erickson, whose contribution to Washington architecture will be the new Canadian Embassy on Pennsylvania Avenue.

Other guests will be Ambassador to West Germany Richard Burt and his wife Gahl Hodges Burt, former Reagan social secretary in the rare role of a White House dinner guest; sportscaster Frank Gifford; "Dynasty" star Catherine Oxenberg; Nicholas Gage, who wrote "Eleni" (Nelligan stars in the movie version); and Chicago Bears running back Walter Payton. To "everybody's" regret in Mrs. Reagan's office, according to Press Secretary Elaine Crispen, is the "regret" sent by another Bear, defensive lineman William (Refrigerator) Perry. Explained Crispen, "Mrs. Perry is very due."

She's performed there for Presidents Johnson and Carter, but tonight's White House appearance by prima ballerina Cynthia Gregory will be her first as a guest. And she and Nancy Reagan will have plenty to talk about.

Gregory, who is marking her 20th anniversary as a ballerina, plans to donate proceeds from her forthcoming Cynthia Gregory Celebration Tour to drug abuse prevention and education groups in 32 American cities. The tour could gross as much as $3 million.

As honorary chairman, Mrs. Reagan will be featured with Gregory on the cover of the tour's commemorative program and on the official poster.

Gregory called the crusade against drug abuse "a very special cause right now and I think it's important that we in the arts combine our efforts to help."

The same White House that gave you Jackie's chocolate mousse, Pat's meat loaf, Jerry's toast and Rosalynn's cheese ball now brings you Nancy's macaroni and cheese. The April issue of McCall's is out with recipes for "President Reagan's Favorite Macaroni and Cheese" plus Hamburger Soup and eight other delicacies from the Reagan kitchen.

Says McCall's: "The President harbors a nostalgic appetite for the earthy, unpretentious meals of his early days in California, and the First Lady makes sure to serve them frequently."

Not personally, of course.

The White House is once again dispatching Nancy Reagan on a diplomatic mission of no small import. Her East Wing office yesterday confirmed she will visit Thailand and Malaysia May 2-5 while President Reagan is in Tokyo for the Economic Summit.

Mrs. Reagan's stated purpose is to inspect drug abuse prevention efforts of the Thai and Malaysian governments. But her visit in Thailand should go a long way toward smoothing still-ruffled feathers over a White House decision in 1983 to scrub Reagan's visit there.

That's the trip during which he was supposed to visit the Philippines. But after Benigno Aquino's murder, White House aides privately cited concerns about congressional opposition to such a visit as well as about Reagan's safety. Along with Thailand and the Philippines, Indonesia also was canceled.

Reagan will make up for it on this trip by flying to the Indonesian island of Bali. Mrs. Reagan comes into the picture by heading off from there to Thailand and Malaysia while the president goes to Tokyo.

In Bangkok, Mrs. Reagan will be the guest of King Bhumibol and Queen Sirikit, whom she and Reagan entertained last year at a private White House dinner. In Kuala Lumpur, the first lady will be the guest of Datin Dr. Siti Hasmah, whose husband is Prime Minister Mahathir bin Mohamad.

Hasmah was among wives of foreign leaders attending Mrs. Reagan's White House conference on drug abuse last April. The two women met again in the fall when Mrs. Reagan hosted a follow-up session for wives accompanying their husbands to the 40th-anniversary celebration of the United Nations. Also at that session was Imelda Marcos, who left her husband at home but brought a military aide to hand out her position paper on drug abuse in the Philippines.