The Irish prime minister says Brexit is fraying relations between Ireland and Britain.
Taoiseach Leo Varadkar said it had also “undermined” the Good Friday Agreement (GFA).
The GFA was signed by political parties in Northern Ireland in 1998 and was aimed at establishing peace after 30 years of The Troubles.
The UK government says “nothing we agree with the EU will risk a return to a hard border”.
The Irish border is one of the biggest sticking points in the Brexit negotiations.
“Anything that pulls the communities apart in Northern Ireland undermines the Good Friday Agreement, and anything that pulls Britain and Ireland apart undermines that relationship,” said Mr Varadkar on RTE’s Marian Finucane programme.
Earlier, the chair of the Republic of Ireland’s Senate Brexit Committee said a return to a hard border threatened the peace process.
Senator Neale Richmond told pro-Brexit Conservative MP Owen Paterson on BBC Radio 4’s Today programme that plans for solving the border dispute using “existing practical systems” was “completely unfeasible”.
Brexit talks have reached an impasse over the EU’s “backstop” plan, which would see Northern Ireland effectively remaining in the customs union and single market unless alternative arrangements were found to prevent a hard border.
The taoiseach’s comments came two days after Nobel peace prize winner and Conservative Lord Trimble accused Mr Varadkar’s government of “riding roughshod” over the GFA.
Lord Trimble, who helped draw up the landmark agreement, said the Brexit process could result in Northern Ireland ending up as part of an “effective EU protectorate”.
Mr Paterson, a former Northern Ireland secretary, reiterated that claim, saying that any backstop which involved the whole UK staying in a customs union would be a “total betrayal” of millions of Leave voters and the 85% of voters at the last general election who backed Tory and Labour manifestos which committed to leaving.
The Irish prime minister said Ireland was entering into a potentially difficult period, even if an agreement was struck.
The UK government said any deal would not ” threaten the arrangements under the Belfast Good Friday Agreement, and we continue to work very closely with the Irish Government on this”.
“We are also working together to ensure the unique bilateral ties between our countries remain strong into the future,” a spokesperson added.
Mr Varadkar also said he had a good relationship with Arlene Foster, leader of the Democratic Unionist Party (DUP).
The DUP and Sinn Féin need to come to an agreement to reinstate the Stormont Assembly, he said.
Northern Ireland has been without an executive since January 2017, when the power-sharing parties – the DUP and Sinn Féin – split in a bitter row.
If there was some clarity on Brexit in the next couple of weeks or months, there would be an opportunity to get the executive up and running again, Mr Varadkar added.
How do you solve a problem like the Irish border?
The UK and the EU both want to avoid a “hard border” between Northern Ireland and the Republic of Ireland, which may include physical checks or infrastructure, but cannot agree how.
A key part of the negotiation is the controversial border “backstop”.
The backstop is a position of last resort, to protect an open border on the island of Ireland in the event that the UK leaves the EU without securing an all-encompassing deal.
What do ‘checks in the Irish Sea’ mean?
In a backstop situation, there could in theory be two types of Irish Sea checks:
- Customs – to make sure the right EU tariffs have been paid
- Regulatory – to make sure goods meet EU safety and quality standards
The government has been adamant it would never accept Irish Sea customs checks.
But it has also been careful not to completely close down the prospect of regulatory checks.