The Irish prime minister has said the UK government needs to provide "a lot more detail" on how it intends to avoid a hard border after Brexit.
Taoiseach Leo Varadkar was speaking at a European Council meeting in Brussels.
He said the UK had used "all the right language" but that was not enough.
The UK has said it is leaving the customs union and single market but that will not mean new border infrastructure.
The Irish border is one of three issues where the EU requires "sufficient progress" before it will talk about transitional arrangements or trade.
Mr Varadkar also asserted that it is likely that a majority of people from Northern Ireland will be Irish and therefore EU citizens after Brexit.
People born in Northern Ireland can choose to be Irish, British or both.
He said he thought people from unionist background would assert their right to Irish citizenship "at the very least for convenience".