During a meeting with Irish Prime Minister Micheal Martin in Dublin, Mr Trump told a reporter he would like to see the reunification of the Republic of Ireland with the UK's Northern Ireland. 'I don't want to cause any problems, but I'd love to see it unified,' Mr Trump said. 'It's going to happen eventually. It may as well happen now. I think it would be a fantastic thing,' he added, noting that 'the UK will have something to say about it, obviously'.
Later, speaking to business leaders at the US ambassador's residence, Mr Trump appeared to soften his position, saying there was 'no good answer' to the reunification question.
The visit angered some Dublin residents who staged protests. Police cordoned off Phoenix Park, where Mr Trump visited the homes of Ireland's prime minister, president and ambassadors. Many in the crowd waved Irish or Palestinian flags and chanted, 'Donald Trump not welcome here' and 'Irish government, shame on you'.
Speaking to Irish national broadcaster RTE, Mr Trump's son Eric defended the visit, saying the golf course in Doonbeg was the largest employer in the area. 'Go down to Morrissey's tavern in Doonbeg and hear from a real Irish person as to whether or not they like President Trump and the Trump family,' he said.
The issue of Irish unity has deep historical roots. Ireland became a self-governing country a century ago, while Northern Ireland remained part of the UK with a Protestant majority. Competing nationalist and unionist visions drove decades of violence known as The Troubles, which ended with the 1998 Good Friday peace accord. Previous US presidents, including Democrat Joe Biden, have avoided commenting directly on the subject.
Mr Trump's visit is the latest example of his blending of official duties and personal interests.