US President Donald Trump has said that he would "love" to see a united Ireland.
Speaking at a news conference with Taoiseach Micheál Martin at Farmleigh House, Mr Trump said that he does not "want to cause any problems".
"But I will tell you, I'd love to see it unified. I have friends on both sides," he said.
"Great people. They're Irish people. How can they be bad, right? I'd love to see it unified.
"I think it would be a great, a great feather in everybody's cap if that happened.
"You know what? The way I look at it, it's going to happen eventually. It may as well happen now."
He said that he and Mr Martin had discussed the issue among other topics in their meeting this morning.
"We were talking about it. You got the right man over here to get it moving," he said in reference to Mr Martin.
He added: "But yeah, I think it would be a fantastic thing.
"Now, the UK will have something to say about it, obviously. But I think it would be a great thing.
"I mean, how can it be bad?"
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A spokesperson for the Taoiseach said Mr Trump's comments on a united Ireland "should come as a surprise to no one".
"Over the years the president has been consistent in his view that he sees unity in Ireland as the optimal outcome," he said in a statement.
The statement added: "The Taoiseach highlighted Ireland’s significant experience in conflict resolution as a result of the peace process, and that this experience could be brought to bear in the Middle East or other conflict areas.
"President Trump was very aware of how violent the conflict in Northern Ireland was, and of the importance of peace."
Later in a speech at the US Ambassador's residence, Mr Trump said that he answered the question on a unified Ireland and he "gave a nice answer. I think we'll read about it tomorrow".
"That’s one of those questions where no matter what you say, no matter how well you say it , there is no good answer," he said.
He added, however, he thought "it would be a very cool thing".
'I think it would be cool', says Trump on Irish unity
SF, DUP and UK govt reacts
Meanwhile, there has been wide reaction to Mr Trump's comments on Irish unity.
Sinn Féin leader Mary Lou McDonald said the conversation on Irish unity "has shifted", and the onus is on the Taoiseach to begin planning.
"We have secured peace, and as we approach the 13th anniversary of the Good Friday Agreement our national journey is now reunification, which is organised, planned and a peaceful transition," she said.
But, she said, the Irish peace process and the role of the US "in this are an example of successful diplomacy and foreign policy where the US has been a critical partner over three decades".
"We want to see President Trump apply the same approach to Palestine and promote a global agenda for peace," she said.
In response, the UK government said Prime Minister Andy Burnham's view remains that there is no majority public support for a referendum and that until that changes, there will not be one.
Last month, Mr Burnham sparked a political row when he said a referendum on unity is "off the table".
Under the terms of the 1998 Belfast/Good Friday Agreement, the UK Secretary of State can decide to hold a border poll in Northern Ireland, and it also places a duty on the office holder to call one if it appears likely a majority would vote for Irish unification.
The people of the 26 counties must also vote in favour to effect constitutional change.
DUP leader Gavin Robinson said Northern Ireland's constitutional future will be decided by the people of Northern Ireland.
In a post on Facebook, he said: "President Trump knows better than most that elections matter.
"Northern Ireland's constitutional future will be decided by the people of Northern Ireland, not by commentary in London, Dublin or Washington.
"Northern Ireland is part of the United Kingdom because that is the settled democratic will of its people."
Mr Robinson added that he is confident Northern Ireland "will continue to choose the UK and the security it offers".
"As unionists we are not interested in the destruction of our country by joining with the Irish Republic," he said.
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Ulster Unionist Party Leader Jon Burrows said: "President Trump, like anyone else in the world, is entitled to express his view on the constitutional future of another country.
"But the only people whose decision counts are the people of Northern Ireland.
"That is the principle of consent, now enshrined in law through the Belfast Agreement, negotiated and delivered by the Ulster Unionist Party."
He added: "But when all is said and done, Northern Ireland's future will be decided by the people of Northern Ireland.
"I am confident that we are safer, more secure and more prosperous within the United Kingdom, and that is where we will remain."