Trump first raised the issue during a meeting with Taoiseach (Irish Prime Minister) Micheál Martin in Dublin on Saturday, saying he would 'love to see a united Ireland' and calling it 'a fantastic thing'. He added that it would happen eventually and 'may as well happen now', telling reporters that Mr Martin was 'the right man over here to get it moving'.
The comments prompted a swift rebuttal from the UK government and Northern Ireland's unionist leaders. British Prime Minister Andy Burnham has maintained that there is not enough popular support for Northern Ireland to leave the UK.
On Sunday, Trump expanded on his remarks, saying: 'It just seems to me you have Northern Ireland and Ireland. It seems that one of the naturals of all time is to put them together.' He sidestepped a question about Scottish independence, saying he would 'save that' for another time.
Trump's visit has included a courtesy call with Irish President Catherine Connolly, a meeting with the Taoiseach at Farmleigh House in Dublin's Phoenix Park, and attendance at the Amgen Irish Open golf tournament. He described the US-Ireland relationship as 'stronger than ever' and 'never as good as it is right now'.