The UK's "competing and even contradictory" Brexit ambitions are presenting difficulties, Ireland's foreign minister has said.
Simon Coveney said all parts of the UK leaving the single market and customs union appeared incompatible with no return to a hard border in Ireland.
If the UK does not stay in the trade-free area, "unique solutions" will be needed for Northern Ireland," he said.
"We are awaiting serious British proposals in this respect," he added.
Mr Coveney said the Brexit process was at a "critical juncture", and the Irish border was one of three issues which required "sufficient progress" before the EU would move on to trade talks, he said.
EU leaders are due to assess progress at a meeting in December.
"Once we get to phase two of these talks, and discussions on the future EU-UK relationship, our neighbours will have no greater friends than the Irish government," Mr Coveney told delegates at a forum in Dublin for Eurofound, an EU agency which monitors labour markets.
At last week's round of Brexit talks, the EU tabled a paper which suggested Northern Ireland will have to continue to follow many EU rules after Brexit if a hard border is to be avoided.
The paper hinted that Northern Ireland may need to stay in the EU customs union if there are to be no checks at the border.
That is something which the UK's Conservative Party and Northern Ireland's Democratic Unionist Party have said they cannot accept as it would effectively create a border between NI and the rest of the UK.