U.S. President Donald Trump expressed effusive praise for Canada and Justin Trudeau Saturday night after Brian Mulroney got a standing ovation for singing When Irish Eyes are Smiling during a cancer benefit at the U.S. President's Mar-a-Lago estate in Palm Beach, Fla.

It's the same song the former Progressive Conservative prime minister sang together with president Ronald Reagan at the so-called Shamrock Summit in 1985 in Quebec City that helped cement a friendship between the two leaders and led to the Canada-U.S. free-trade deal.

Mr. Mulroney and his wife, Mila, had been invited to dine at the same table as Mr. Trump and his wife, Melania, and Boston power couple Michele and Howard Kessler. The event was a dinner and dance fundraiser for the Dana-Farber Cancer Institute, which raised $2.3-million (U.S.) from the wealthy crowd of 800.

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Canadian songwriter and composer David Foster, who was the main entertainer, invited Mr. Mulroney to the stage, noting that St. Patrick's Day is right around the corner and asked him to sing When Irish Eyes Are Smiling.

When he took the microphone, Mr. Mulroney joked: "Mr. President, I hope this doesn't wreck Canada-U.S. relations for decades to come."

U.S. President Donald Trump expressed fulsome praise for Canada and Justin Trudeau Saturday night after Brian Mulroney got a standing ovation for singing When Irish Eyes are Smiling during a cancer benefit at the U.S. President’s Mar-a-Lago estate in Palm Beach, Fla.

The former prime minister got a standing ovation led by the Trumps, according to attendees.

When he got back to the Trump table, the President told Mr. Mulroney that "relationships are just great between Canada and the United States. Justin had a terrific trip down to Washington."

Mr. Mulroney has taken on the role of Mr. Trudeau's unofficial emissary to the United States, using his long-time personal connection to Mr. Trump and other members of his cabinet, as well as the President's outside business advisory councils, to help shield Canada from America First protectionist policies.

Mr. Mulroney has developed a close bond with Mr. Trudeau despite belonging to different political parties. Pierre Trudeau and Mr. Mulroney were sworn enemies. Mr. Mulroney has never forgiven the elder Trudeau for his intervention that helped kill the Meech Lake accord, which Mr. Mulroney had championed to address Quebec's isolation from the 1982 Constitution.

Since Mr. Trump's election, Mr. Mulroney and former Canadian ambassador to Washington Derek Burney have acted as informal advisers on how to handle the Republican-led Congress and the Trump White House and cabinet secretaries.

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A source said the President's son-in-law, Jared Kushner, has said in private that the White House has found it easy and professional to work with Mr. Trudeau's team of advisers that include principal secretary Gerald Butts, chief of staff Katie Telford and Canadian Ambassador to the United States David MacNaughton.

Mr. Trudeau won a major achievement from his visit to the White House when Mr. Trump said he was interested in only "tweaking" the North American free-trade deal as it governs commerce with Canada. Mr. Trump's focus is on rebalancing the big trade surplus that Mexico has with the United States as well as building a wall and confronting illegal immigration from that country.

Canada and the United States are expected to hold bilateral negotiations on NAFTA to reach agreements on issues such as e-commerce, precargo clearance and making it easier for high-tech professionals to work in either country.