"I mean she's got to do what she's got to do", he said at the White House as he welcomed Irish Prime Minister Leo Varadkar for an early St. Patrick's Day celebration.
Speaking on Thursday, Trump then took aim at the possibility of a second referendum on leaving the European Union, which he deemed "unfair" on those who had campaigned successfully for Brexit in 2016.
I gave the Prime Minister my ideas on how to negotiate it and I think you would have been successful. "I hate to see everything being ripped apart now".
On the prospect of another referendum, Mr Trump said: "I don't think another vote would be possible because it would be very unfair to the people that won".
US President Donald Trump said on Thursday that he was "surprised at how badly" the Brexit negotiations have gone.
"I particularly want to thank you for your help with the plant in the west of Ireland where hundreds of jobs were threatened as a result of the Russian sanctions", he stated. "I stand here leader of my country, flawed and human, but judged by my political actions and not my sexual orientation, my skin tone, gender, or religious beliefs", he said.
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The issue of the United Kingdom border with the Republic of Ireland is one of Brexit's most complex points, noted Trump.
Trump has previously been critical of May's Brexit deal, warning that it could "kill" a bilateral trade agreement between the U.S. and the UK.
The Irish PM said the most pressing issue facing his country was how to settle questions about the future of the border between Ireland, an European Union member, and Northern Ireland, which won't be. "I regret that Brexit's happening".
Ireland is at the center of the practical discussion of how Britain would separate itself from the European Union and its open movement of goods and people, because the only affected land border runs between Ireland and Northern Ireland.
"I predicted it was going to happen, and I was right".
The Taoiseach said: "Well, we have a different opinion. But they're going now and that's their decision", Varadkar said, adding that the negotiations shouldn't cause any problems in Northern Ireland. He said: "If they don't talk to us, we're going to do something pretty severe economically".
After their private meeting, Mr Varadkar said it had been an opportunity for him to set out Ireland's position on Brexit, "particularly when it comes to the peace process".