Leo Varadkar has said he believes a Brexit deal is still possible, but that the PM's current proposals do not "form the basis for deeper negotiations".
The taoiseach (Irish PM) said a deal must be agreed ahead of the European Council summit in two weeks.
Speaking on Saturday, he also said he hoped to meet Boris Johnson next week.
Mr Johnson's proposals would see NI stay in the European single market for goods, subject to approval every four years by the Northern Ireland Assembly.
Northern Ireland would leave the EU customs union along with the rest of the UK - resulting in new customs checks.
Mr Varadkar told the Irish national broadcaster RTÉ News there was still time for the UK government to put forward further proposals.
"I think a deal is still possible," he said.
"It is possible at the European Council summit in two weeks' time but the current position as of today is the European Union, including Ireland, doesn't feel that the proposals put forward by Prime Minister Johnson yet form the basis for deeper negotiations."
The taoiseach said that Friday would be a reasonable deadline for new proposals, but that it was not his job to set one and he did not want to set a false deadline.
On Friday his deputy Simon Coveney described Mr Johnson's latest Brexit proposals as a "step forward" but insisted that further change is needed to avoid customs checks.
Talks will resume on Monday after the EU said the UK's proposed alternative to the Irish backstop could not be the "basis" for a legally-binding treaty.
The UK has said it would work on the details before then but there was "no path" to a deal without alternative arrangements in Northern Ireland.
What is in the plan?
The PM's Brexit plan set out details of a replacement for the Irish border "backstop" in the current Brexit agreement.
The backstop is the controversial "insurance policy" that is meant to keep a free-flowing border on the island of Ireland but which critics - including the PM - fear could trap the UK in EU trading rules indefinitely.
Under Mr Johnson's proposals, which he calls a "broad landing zone" for a new deal with the EU:
- Northern Ireland would leave the EU's customs union alongside the rest of the UK, at the start of 2021
- But Northern Ireland would, with the consent of politicians in the Northern Ireland Assembly, continue to apply EU legislation relating to agricultural and other products - what he calls an "all-island regulatory zone"
- This arrangement could, in theory, continue indefinitely, but the consent of Northern Ireland's politicians would have to be sought every four years
- Customs checks on goods traded between the UK and EU would be "decentralised", with paperwork submitted electronically and only a "very small number" of physical checks
- These checks should take place away from the border itself, at business premises or at "other points in the supply chain"
The government is also promising a "New Deal for Northern Ireland", with financial commitments to help manage the changes.