The Irish government has cast doubt on suggestions that Northern Ireland’s Stormont assembly could provide the key to unlocking the Brexit impasse.

Simon Coveney, the deputy prime minister, expressed scepticism about reports that the Democratic Unionist party might sign up to a Northern Ireland-only backstop for the Irish border if the devolved legislature had a veto on future EU rules applying in the region.

“There is certainly a concern at an EU level that a devolved institution in Northern Ireland could have a veto over how the single market operates or a border on the single market operates, so it’s not as straightforward as some people are suggesting,” said Coveney after a two-day party “think-in” in Cork.

The taoiseach, Leo Varadkar, said there was ongoing contact with the DUP and other parties in Northern Ireland, and further talks at government level are believed to be pencilled in for next week.

But he said he was not aware of any change in the unionists’ position over their refusal to accept any checks in the Irish Sea as a means of keeping the Irish border invisible.

The DUP leader, Arlene Foster, dismissed reports it had dropped its red lines and would accept EU regulations in certain circumstances as nonsense.

Coveney said there were still no written proposals on the table and this was “a source of real frustration”.

“I think you saw that yesterday when you saw [Michel[ Barnier say there is no basis for the reopening of a serious negotiation because there are no proposals coming forward so the next step here is the British government has got to bring forward real proposals in writing that can do the job that the backstop does if they are proposing an alternative,” he said.

Under the Northern Ireland-only backstop, regulations in the region would remain aligned to those of the EU even if the UK diverged from EU rules in key areas post-Brexit. This would allow animals, food and other goods to be traded across the border without customs or health checks.

The DUP has denounced this as a move to break up the UK, but sources have said they might reconsider if the backstop came with its own backstop – the so-called “Stormont lock”.

This would allow the assembly to have a veto on the EU rules that would apply when and if the rest of the UK diverged from EU laws. The DUP dismissed the lock at the time as “cosmetic and meaningless” because it was not legally binding.

Some, however, believe that governance by Stormont could be part of the solution if it were crowbarred into the withdrawal agreement.

The frustration comes after a week of shuttle diplomacy in which Boris Johnson flew to Dublin for his first face-to-face meeting with Varadkar since he became prime minister.

In what appeared to be a shift away for his “do or die” no-deal policy, Johnson said his overwhelming preference was for a deal, claiming it would be a “failure of statecraft” for all concerned if they were unable to strike a deal.

His change in tone led many to believe the mood music had changed and space had opened up for a deal centring on the original Northern Ireland-only backstop.

Phil Hogan, the EU’s new trade commissioner and an ally of Varadkar, said he detected a shift in position offering a glimmer of hope of a deal.

“I also note that the British prime minister has moved away from his position … where he’s now prepared to look at divergence of certain rules and regulations on the island of Ireland vis-a-vis the United Kingdom,” he said.

Many believe this could survive only if the backstop were rebranded because it is now considered a toxic term.

Hogan told RTE: ”There are constitutional issues that are already in the withdrawal agreement that might have to be improved upon if this is a request that’s made. Of course we can look at it.”