The Taoiseach Enda Kenny says the European Union needs to prepare for the prospect of a united Ireland, as part of the talks on Brexit.
Mr Kenny says the transfer of Northern Ireland back to the Republic is something that must be considered.
The Taoiseach pointed out that East Germany did not have to apply for EU membership in 1990 when Germany was reunited.
He says a similar prospect cannot be ruled out in Ireland.
Mr Kenny was speaking to reporters at the MacGill Summer School in Donegal.
His comments come after the UK Brexit minister said the border between Ireland and the UK would be one of the "really challenging issues" to deal with.
David Davis said: "One of our really challenging issues to deal with will be the internal border we have with southern Ireland."
"We're not going to go about creating other internal borders within the United Kingdom" he added, seemingly referring to Scotland.