The second annual Shamrock Summit got under way at the White House last night and, acid rain or no acid rain, it was "Ron" and "Nancy" and "Brian" and "Mila" by the time the toasts rolled around."

"I'm sorry," Canadian Prime Minister Brian Mulroney remarked later with a knowing grin of his first-name reference to President and Mrs. Reagan. "I just didn't think."

Actually, it was Reagan who started it when he described his first Shamrock Summit with "Brian" in Canada almost a year ago to the day, "one of the most enjoyable St. Patrick's Days in my memory. And I assure you, my memory goes back a long way."

After-dinner toasts by both leaders touched on trade liberalization, defense and a manned space station project. Only Reagan brought up acid rain. Mulroney pointedly avoided the subject.

"No, no, no, that's for tomorrow," Mulroney said in the Blue Room, lapsing into French to emphasize his point. "Chaque chose dans son temps -- there's a time for everything. I didn't think our good friends in Washington needed me to inflict acid rain on them one more time."

He said Reagan is supposed to respond today to a report on the problem of acid rain developed by the special envoys each appointed.

"So I hope there will be something positive. I've already endorsed it, and I'm hoping the president will, too. This will be a major step forward," Mulroney said.

If Reagan had decided his position, he kept his own counsel. The first get-together in Quebec last year established what Reagan called "a new partnership -- we continue today fully recognizing the long-term stakes. We can work together to resolve the issue of acid rain as we've worked together to solve so many environmental issues before."

Later, surrounded by his glittering guests from the worlds of fashion, film and foreign lands, Reagan denied a CBS report that the U.S. Navy is planning operations near Libya in the Gulf of Sidra.

"The exercise that is going to be done -- I can't even tell you when, I don't pay that much attention to it -- is the same exercise we have done every year," Reagan said. "We can't just have a Navy unit sit there doing nothing."

Asked by reporters if there was a special message he wanted to send Muammar Qaddafi, Reagan said, "I'm not interested in sending him anything. He wouldn't be able to understand it."

On aid to Nicaraguan rebels, Reagan declined a turn at the crystal ball. "As a radio sportscaster, I've never predicted the end of a game, I've never said I'd win an election. And I just want to tell the world it's very important we provide this aid."

Not everything was so serious. Chicago Bears running back Walter Payton was agog at the glamorous and famous surrounding him, and at one in particular whom he accidentally bumped into -- the president.

"I'm so star-struck with all the people," Payton explained.

The fashion parade seemed more sparkling than usual. Mila Mulroney's dress was sparkling purple, Pat Buckley's was sparkling black and silver, and Nancy Reagan's sparkling brown and gold outsparkled them all.

"I think the White House is one of the few places left in our world that floor-length gowns work," said fashion designer Norma Kamali. "I don't really know people who wear floor-length gowns. At this and a wedding, it's okay."

Kamali wore a navy-blue suit with a knee-length skirt and short blue anklets over her sheer blue stockings.

Maureen Reagan, in green velvet, said she hasn't read her half-sister Patti Davis' novel, "Home Front," which strikes close to the Reagan home front, in some reviewers' opinion. Asked if she thought Patti should have written the book at this time, Maureen replied tartly, "I don't tell people what to do."

Marty Davis, the photogenic congressional wife whose revealing pose in Dossier magazine catapulted her to celebrity status, wore a sky-blue calf-length dress. She said she has been talking with local television stations about doing "various and sundry shows."

She said she had no plans to pose in any more skimpy bathing suits. "We went in December on vacation to Florida but nobody looked."

"Actually," said her husband, Rep. Robert W. Davis (R-Mich.), "I have better legs."

The four-course dinner featured angel-hair pasta with seafood, supreme of chicken vol-au-vent, salad and chevre cheese and pistachio marquise with petits fours.

There was no birthday cake, although, as Reagan told guests following dinner, "in just two days Brian will be 47 years old and, kid, I wish you the very best.

"Every time they bring out my birthday cake, the top of it is beginning to look more and more like a torchlight parade," Reagan said.

In the tradition of Irish pols, Mulroney displayed a penchant for telling a joke.

"I suppose, Mr. President, Canadians most admire of all things your uncanny ability to forecast the future. When you became aware of what television was going to do to the movies, you decided to try something easy. You chose politics, and we're glad you did," Mulroney said.

Entertainment was by concert pianist Rosalyn Tureck, playing Bach. Publisher William Buckley was so carried away by it that he, alone, gave her a standing ovation.

"You're magic," Reagan told her.

The Mulroneys went to their hotel soon after, and others went to the dance floor. Maureen Reagan and Secretary of State George Shultz did a couple of twirls, and the Reagans milled among the guests. Among them were Fiat Chairman Giovanni Agnelli and Donna Marella Agnelli, "Dynasty's" Catherine Oxenberg, ballerina Cynthia Gregory, actress Kate Nelligan, actor Christopher Plummer and authors Arianna Stassinopoulos and Nicholas Gage.

And the Aga Khan family seemed to be everywhere -- Prince Karim with Princess Salimah and Prince Sadruddin with Princess Catherine.

Earlier in the day, Shultz hosted a luncheon for the Mulroneys, and things were on a first-name basis there too.

"One of the things we share is an extraordinary friendship," Shultz said of Americans and Canadians. "Our bilateral relationship is extensive, complicated and rewarding."

For Mulroney, Shultz's luncheon provided a forum to keep in touch with the folks back home via a battery of Canadian and U.S. television cameras. In recent months, his government has been hit by one annoying little scandal after another, the latest one involving wiretapping.

He didn't get into things like that though when he took the microphone to announce -- first in English, then in French -- that he had told Reagan earlier in the day that Canada would participate in the construction and operation of the U.S. manned space station.

"Canada will design and build a key part of this station," Mulroney said. "The mobile servicing center will play an active role in operating and managing the space station once this facility becomes operative in the next decade."

At the luncheon, too, he avoided any mention of acid rain and the ongoing debate between the United States and Canada about environmental pollutants. But Shultz, who called for "steely determination" in finding a solution, referred to it in general terms.

Shultz told of "a balanced report," produced by American and Canadian representatives, "which has focused our attention on the issue most constructively."

The other guests were a me'lange of business leaders, legislators, media personalities and past and present administration officials. They included Donald Kendall of PepsiCo Inc., newsman Roger Mudd, Undersecretary of the Treasury Richard Darman, Federal Reserve Chairman Paul Volcker and Sen. Russell Long (D-La.).

Guests at last night's White House dinner:

Allan E. Gotlieb, ambassador of Canada, and Sondra Gotlieb

Robert Anderson, M.A. Hanna Co., and Marjorie Anderson

Jonathan Deitcher, Dominion Securities Pitfield, and Dianne Deitcher

Moses Deitcher and Phyllis Deitcher

Paul Desmarais, Power Corp. of Canada, and Jacqueline Desmarais

F. Ross Johnson, president, R.J. Reynolds Industries, and Laurie Johnson

Robert Shea, Shea Financial Group, and Gertrude Shea

Prince Karim Aga Khan and Princess Salimah Aga Khan

Prince Sadruddin Aga Khan and Princess Catherine Aga Khan

Giovanni Agnelli, chairman, Fiat, and Donna Marella Agnelli

John M. Albertine, vice chairman, Farley Industries, and Mona Albertine

Nelson Benton and Mildred Benton

Winton M. Blount Jr., chairman, Blount Inc., and Carolyn Blount

U.S. Secretary of Health and Human Services Otis Bowen and Rose Bowen

William F. Buckley Jr., editor in chief, The National Review, and Patricia Buckley

L. Keith Bulen, International Joint Commission for the U.S. and Canada, and Sandra M. Donovan

Chief Justice Warren Burger and Elvera Burger

Richard R. Burt, U.S. ambassador to the Federal Republic of Germany, and Gahl Hodges Burt

Vice President George Bush and Barbara Bush

James M. Collins, former U.S. representative from Texas, and Dee Collins

Rep. Robert W. Davis (R-Mich.) and Marty Davis

Sidney Earl Dove, chairman, American Trucking Association, and Margaret Dove

Arthur Erickson, architect, new Canadian Embassy

Raymond F. Farley, president of S.C. Johnson & Sons Inc., and Mary Farley

Rafael Franchi, chairman, Republican National Hispanic Assembly for Virginia, and Mercedes Franchi

Nicholas Gage, author, "Eleni," and Joan Gage

Sen. Jake Garn (R-Utah) and Kathleen Garn

Frank Gifford, sports announcer, and Astrid Gifford

Cynthia Gregory, ballerina, and Hilary Miller

Michael Huffington

R.L. Ireland III, Brown Brothers Harriman & Co., and Jacqueline Ireland

Norma Kamali, fashion designer, and Crist L. Zois

Jerome Karle, 1985 Nobel Prize for Chemistry winner, and Isabella Karle

William Lansdale and Marianthi Lansdale

Rep. Romano L. Mazzoli (D-Ky.) and Helen Mazzoli

Sen. Mitch McConnell (R-Ky.) and Pam Schultz

Larry Mizel, Mountain Financial Services, and Carol Mizel

Allen E. Murray, president and chairman, Mobil Corp., and Patricia Murray

Kate Nelligan, actress

Thomas M.T. Niles, U.S. ambassador to Canada, and Carroll Niles

Burl Osborne, president-editor, Dallas Morning News, and Betty Osborne

Catherine Oxenberg, actress, "Dynasty"

Mollie Parnis, fashion designer

Walter Payton, running back, Chicago Bears, and Connie Payton

Ronald O. Perelman, chairman, Revlon Inc. and MacAndrews & Forbes Group Inc., and Claudia Perelman

Christopher Plummer, actor, and Elaine Plummer

Adm. John M. Poindexter, assistant to the president for national security affairs, and Linda Poindexter

Edmund T. Pratt Jr., chairman, Pfizer Inc., and Jeanette Pratt

Maureen Reagan and Dennis Revell

Donald T. Regan, chief of staff to the president, and Ann Regan

Rozanne L. Ridgway, assistant secretary of state for European and Canadian affairs, and Capt. Theodore Deming, USCG

Selwa Roosevelt, chief of protocol, and Archibald B. Roosevelt Jr.

Rep. Toby Roth (R-Wis.) and Barbara Roth

Albert E. Schwabacher and Cynthia Schwabacher

Secretary of State George P. Shultz and Helena Shultz

Arianna Stassinopoulos, author

Alfred Stern, director, New York Film Festival, and Joanne Stern, president, International Council, Museum of Modern Art

Rosalyn Tureck, concert pianist, and Diana Kitch

Adm. James D. Watkins, chief of naval operations, and Sheila Watkins

Thomas Wendel, professor of history, chairman of Beethoven Society, and Charlotte Wende