If border posts return to Northern Ireland they will be attacked, according to the Taoiseach.
Leo Varadkar says they'll become natural targets for people if they have to be re-established post-Brexit.
But he believes there won't be a return to the violence of the past in Northern Ireland.
Mr Varadkar says certain tensions could be re-ignited by a hard border.
"I don't anticipate a return to violence like the kind we had 20 or 30 years ago.
"But if you do have physical infrastructure - if you do have cameras, and signs and border posts - those things will become targets.
"And then what do you do? Do you accept that, or do you then put in a guard to stop them doing that.
"And that's why it might all escalate and go the wrong way".
Taoiseach Leo Varadkar @campaignforleo meeting with @GovAbbott of Texas & First Lady Cecilia Abbott - who has Irish heritage - in Austin this morning #SPD2018 #StPatricksDay pic.twitter.com/qLC2IxV1Zc
— Embassy of Ireland USA (@IrelandEmbUSA) March 11, 2018
LGBT rights
The Taoiseach says he'll raise LGBT rights when he meets US Vice President Mike Pence this week.
He says he's disappointed the US is no longer a leader in the gay rights campaign.
Vice President Pence contests the charge that he's in favour of conversion therapy to change someone's sexual orientation.
The Taoiseach says it's an area he intends to discuss with the US leaders.
"What I intend to say is that for the cast majority of people around the world - including people from gay, lesbian and transgender backgrounds - we've 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 rights movement began, the first place where gay people fought back".
Later the Taoiseach will visit a Native American tribe in Oklahoma, who donated money to Ireland during the famine.
The Choctaw Nation were driven off their lands in the early 1800s and re-settled in Oklahoma after a vicious drive across states, known as the Trail of Tears.
Despite their own plights, just 16 years later the Choctaw fundraised for the Irish people - sending $170 in famine relief money to Ireland.
That sum would be worth around €4,000 today.
