Donald Trump and Leo Varadkar disagreed on Brexit and the US President used the Taoiseach's Washington visit to rail against EU trade policy with the United States.
Amid this week's Westminster turmoil, Mr Trump criticised British Prime Minister Theresa May's handling of the UK's departure from the EU.
In a somewhat chaotic Oval Office press conference with the Taoiseach, Mr Trump said he intends to visit Ireland this year, renewed his pledge to build a border wall with Mexico and mocked his potential 2020 presidential rival, Irish-American politician Beto O'Rourke.
While bilateral relations and Irish undocumented immigrants in the States were on the agenda for Mr Varadkar's Oval Office meeting with Mr Trump, Brexit dominated their talks.
Just hours before Mr Varadkar's visit, Mr Trump tweeted that he looked forward to making a trade deal with the UK. It came as the process of the US and EU reaching a trade deal faces continuing difficulties.
Mr Trump was asked if he was still a Brexit supporter.
"It wasn't that I was a supporter. I predicted it was going to happen," he said.
He said he was surprised how badly the Brexit talks had gone, adding: "I gave the prime minister my ideas on how to negotiate... she didn't listen to that and that's fine.
"She's got to do what she's got to do but I think it could have been negotiated in a different manner.
"I hate to see everything being ripped apart right now."
The President said he didn't think a second referendum would be possible, adding that the "issue on the border of Ireland is one of the most complex points".
He said it is the UK's decision and the most important thing for Ireland is to avoid a hard border and protect the Northern Ireland peace process.
Mr Varadkar also said Ireland wants frictionless trade with Britain and believes in free trade and that, while it may be years before the UK "sorts itself out", the EU is "available to talk trade with the US".
This prompted Mr Trump to complain about how the US was treated in trade talks with the EU and warned of tariffs being slapped on EU products.
"If they don't talk to us we are going to do something pretty severe economically. We're going to tariff a lot of their products coming in because the EU treats us very, very unfairly," he said.
After their private meeting, Mr Varadkar said: "Needless to say we have very different views on Brexit as to whether it's a good thing or a bad thing."
He said it was an opportunity for him to set out Ireland's position, "particularly when it comes to protecting the peace process".
The traditional St Patrick's Day shamrock ceremony took place in the White House last night. Earlier Mr Trump said he and Mr Varadkar are becoming "fast friends" and he said the Taoiseach is a "very popular man doing a wonderful job".
On his plans to visit Ireland, Mr Trump said: "I'll be coming at some point during the year. I missed it last time but I would have loved to have been there. It's a special place."
He said he has a "very warm spot" for Doonbeg, where he owns a golf resort.
"It's just a great place," he added.