Donald Trump has triggered a diplomatic row after backing a united Ireland during a visit to Dublin, wading into a politically sensitive subject as he embarked on a two-day Irish visit that blurs the lines between his presidency and personal business interests.

Speaking to reporters on Saturday, the US president claimed Irish reunification was inevitable, adding it "may as well happen now".

His comments brought a stern rebuke from the Conservatives in the UK while Downing Street pointed to Prime Minister Andy Burnham's rejection of an immediate border poll earlier in the week.

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