Trump was among the spectators during a 48-hour visit to Ireland. He arrived on Saturday after meeting Irish leaders in Dublin, forcing play to pause while his presidential helicopter landed between a fairway and a nearby field of cows.
Lowry, who described Saturday's interruption as "an insane day", had taken control with an eight-under course record the day before and came one shot from matching it on Sunday, draining an 18th hole birdie to finish with a 23-under-par total.
"He was unbelievable. He's a great guy," Trump said of Lowry, before further delighting the home fans by saying he was going to take tariffs off Irish whiskey.
Lowry, who fought back tears as he paid tribute to his family at the end of his round, thanked Trump and Irish Prime Minister Micheál Martin for being at the event, saying it was amazing to have a US president in attendance.
The 2019 British Open champion's record 11-shot victory came 17 years after he became just the third amateur player to win a European Tour event at his home tournament.
Watching president
Trump, wearing a suit and white USA cap, greeted Lowry on the first tee and watched from a few feet away as he teed off. Trump told reporters that Lowry was "playing great." Lowry later said it was "a bit mad" to have the U.S. President watching on.
While thousands of anti-Trump protesters marched through Dublin on Saturday, the president was cheered heartily throughout Sunday and during a brief appearance in the club bar in the adjoining hotel where he stayed on Saturday night.
Trump watched the action on both days from an 18th-green balcony alongside his sons, Don Jr. and Eric and some of his grandchildren. He was joined by US major champions Brooks Koepka and Patrick Reed, who finished in second place.
Trump has always received a warm welcome in the tiny County Clare village of Doonbeg, where the golf resort he bought in 2014 is a major employer for the local population of 300. Some fans wore the president's bright red 'Make America Great Again' hats.
"He will have detractors and people are not very happy politically-wise, but I just feel it's a golf event," said 69-year-old retiree Kathryn Murphy, welcoming Trump's attendance and support for the local economy.
"I'm sure he has 1 million and one things to do, and it's a fantastic honour to have him here because America is a very good country to Ireland."
