He's on day two of a visit, which he is spending at Trump International Golf Links in Doonbeg, County Clare, for the Irish Open.
On Saturday, Trump told reporters he would love to see a united Ireland while at a meeting with the Taoiseach (Irish PM) Micheál Martin in Dublin.
Speaking on Sunday at his Doonbeg golf resort, he said it would be natural to put Ireland and Northern Ireland "together".
He spoke to reporters after watching the tournament leader Irish golfer Shane Lowry tee off.
