Germany’s foreign affairs minister is to fly to Dublin on Tuesday for Brexit talks as relations with Ireland intensify in an attempt to find a “fix” that will help Theresa May get the EU withdrawal agreement ratified.

Heiko Maas will address an annual gathering of Ireland’s global diplomatic corps and take part in an unofficial fourth round of talks between Ireland and German leaders since Thursday. He will make the address in English, with a large German media contingent accredited, a reflection of how significant his speech is deemed back in Berlin.

Last week the taoiseach, Leo Varadkar, had a lengthy telephone call with Angela Merkel. He then flew to Munich to address a meeting of her coalition partners, the CSU, and on Friday met the Germany chancellor’s successor as CDU leader, Annegret Kramp-Karrenbauer, for discussions on Brexit and the future of Europe.

The emerging Irish-German nexus on the Irish border backstop “fix” is being seen as significant in Irish political circles, where people also point to the fact that Varadkar speaks German and has a good working relationship with Merkel. They point out it was Merkel, not the taoiseach, who requested the phone call with Varadkar last Thursday.

The talks lasted 40 minutes and were, according to Varadkar, “an opportunity to kind of brainstorm a bit as to what we could do to assist prime minister Theresa May in securing ratification of the withdrawal agreement”.

But informed EU sources say Brexiters should not raise their hopes of a reopening of negotiations. The “fix” will be further details in the political declaration on the future relationship and not the 585-page withdrawal agreement. “That is locked,” said one EU source.

There is deep frustration that the British cannot see how far the EU went to break the impasse on the Irish border talks, yielding to May’s demands for a UK-wide customs arrangement. One EU source said: “The EU was totally opposed to this in 2017 and again in March and June in 2018. It then emerged out of the tunnel in the autumn as the solution, but the Brexiters did not see it for what it was – a major concession.

“They are now looking for more concessions, but they just can’t be given. The Brits banked this major concession and just did nothing with it. People can’t understand why it wasn’t sold as a victory for May.

“The EU cannot now give another concession ahead of the vote because if the deal isn’t ratified, it means any new concessions will simply be banked again to no benefit at all. It would be pointless.”

No-deal planning is being stepped up in Ireland, where the cabinet is being briefed on 15 January on medicines and transport arrangements.