If border posts return to Northern Ireland they will be attacked, according to the Taoiseach.
Leo Varadkar says they will become natural targets for people if they have to be re-established post-Brexit.
But he believes there will not be a return to the violence of the past in Northern Ireland.
Speaking in the US at the start of a week-long St Patrick's Day visit, Mr Varadkar said certain tensions could be re-ignited by a hard border.
"I don't anticipate a return to violence like we had 20 or 30 years ago but if you do have physical infastructure like cameras and signs and border posts then those things will become targets and people will vandalise them, people will remove them, people won't accept this border in their towns and villages and parishes," said Mr Varadkar.
"Then what do you do? Do you accept that or do you put in a guard to stop them doing that?
"And that's why it might all escalate and go the wrong way."
Meanwhile,the Taoiseach has also said that he will raise LGBT rights when he meets US Vice President Mike Pence this week saying that he is disappointed the US is no longer a leader in the gay rights campaign.
Vice President Pence contests the charge that he is in favour of conversion therapy to change someone's sexual orientation.
Mr Varadkar says it is an area he intends to discuss with the US leaders.
"What I intend to say is that for the vast majority of people around the world including people from gay, lesbian and transgender backgrounds, we have always seen America as a beacon of freedom.
"This is the land of the free, the home of the brave. This is where the LGBT movement began, the first place where gay people fought back."