Taoiseach Micheál Martin has tested positive for Covid-19.
It means he will be unable to meet US president Joe Biden in person later on Thursday as part of the traditional shamrock ceremony.
Mr Martin had been due to speak at the 30th anniversary of the Ireland Funds Gala dinner on Wednesday evening.
He left the gathering after the positive result was confirmed.
Mr Martin’s speech was instead given by Irish Ambassador to the US, Daniel Mulhall.
“I know you will all join with me in wishing the Taoiseach a speedy recovery,” he told the audience in Washington.
It remains unclear how the White House St Patrick’s Day celebrations will now be handled.
Early indications on Thursday suggested the event may take place virtually, but nothing has been confirmed.
It also casts doubt on whether the Taoiseach can return to Ireland for a national day of remembrance on Sunday to commemorate those who died during the Covid-19 pandemic.
Just before the positive Covid-19 test result was confirmed, Mr Martin had been listening to remarks from US president Joe Biden, who addressed the dinner on Wednesday evening.
Mr Biden had welcomed the Taoiseach to Washington and said he was looking forward to renewing their friendship on Thursday.
Mr Biden, who was greeted with a lengthy round of applause and repeated cheers from the audience, was in fine spirits as he addressed the gathering.
He welcomed Taoiseach Micheál Martin and recalled meeting him at the White House shamrock ceremony in 2009, when Mr Martin was foreign affairs minister and Mr Biden was vice-president.
“We’ve come full circle.
“You’re Taoiseach, I’m President – what the hell are we going to do?”
Last year, Mr Martin had been forced to meet with the US president virtually due to the pandemic.
All parties had been looking forward to an in-person meeting this year.
In front of an audience that included Speaker of the House of Representatives Nancy Pelosi and UK Secretary of State for Northern Ireland Brandon Lewis, Mr Biden spoke passionately about his Irish heritage but did not shy away from the multiple crises facing the world.
He said that Ireland was the only country in the world that was “nostalgic for the future”.
“But of course, that means dealing with the present.
“In our time we’ve seen more change and challenges, I believe, than any time in generations. A once-in-a-century pandemic and economic unease and anxiety, existential threats, a climate crisis, and what we see today in Ukraine, an unprovoked war of aggression and just a vicious, vicious, vicious treatment.
“The march of reactionary forces of nationalism, nativism and isolationism, of autocracies – not just in Europe. We’ve seen it around the world. We’ve seen it here in America.
The president spoke the alternative US tradition of “strength, necessity and endurance of democracy”.
President Biden also said that Ireland, the home of his emigrant ancestors, was now a leader on the global stage.
He described the Republic of Ireland as a “global force in culture and in the arts”.
He added: “Leaders on the world stage, members of the United Nations Security Council, a country with a past that tugs at our hearts and a future that’s going to shape the world.”
Mr Biden had ended his remarks by recollecting that both his and former president Barack Obama’s distant grandfathers had left Ireland in the middle of the 19th century.
The two shoemakers, the president told the room, might have known each other and left Ireland within weeks of one another.
“They left everything behind for an uncertain future.
“But I wonder, in all their dreams that they could ever have dreamt, that each of their grandsons would one day be president of the United States of America.
“That’s America.”
In his speech, the Taoiseach had been due to discuss the conflict in Ukraine, the importance of the Good Friday Agreement and his Government’s Shared Island initiative.
Delivering the address instead, the Irish Ambassador said that the conflict in Ukraine served as a reminder of how “fragile peace can be”.
Praising the support of US politicians in the peace process, Mr Mulhall said: “We mustn’t lose sight on the fact that peace and stability are organic and can grow and prosper, but they can also wither. They need tending and willing guardians.
“The implementation of the Good Friday Agreement is therefore a continuous process. Its impact has been transformational and yet work to reconcile communities and heal old wounds by necessity continues.
“Your commitment to that cause is as important today as it ever was. It has already borne great fruit.”
Mr Mulhall said that the Irish Government was committed to working with the UK government to “protect the achievements of the peace process, including more recently in response to Brexit”.
