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 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 program.
Mr Varadkar also said he had a good relationship with Arlene Foster, leader of the Democratic Unionist Party (DUP).
He said the DUP and Sinn Féin needed to come to an agreement to get the Stormont Assembly up and running again.
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.
Mr Varadkar said that 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.
He also said Ireland was entering into a potentially difficult period, even if an agreement was struck.