Tusk was due to meet with Prime Minister Leo Varadkar in Dublin on Friday afternoon, ahead of a Monday deadline by which the EU has demanded progress if Brexit talks with Britain are to move forward.
![Flooded street of Dubai [PAP]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Flooded street of Dubai [PAP]")
All sides agree there should be no return to physical border checks between Ireland and British-controlled Northern Ireland after Brexit, which might upset the fragile peace in the region.
But Dublin's demand for written guarantees from Britain has proved an obstacle to an early agreement, threatening to delay the wider negotiations and causing tensions with London.
Speaking to reporters ahead of Tusk's visit, Foreign Minister Simon Coveney said his government was being "positive", but added: "We're not there yet."
Dublin wants "reassurance" that regulations on issues such as food safety and animal welfare would be maintained in Northern Ireland, to avoid damaging cross-border trade once Britain leaves the EU's single market and customs union.
"We can't be asked here to leap into the dark by opening up a phase two discussion in the hope that these issues might be resolved," Coveney told BBC radio.
DUP threatens UK government support
A British newspaper reported on Thursday that the two sides were close to a deal that would avoid regulatory divergence between Ireland and Northern Ireland, even if the rest of Britain moved away from EU rules.
The Times said this would involve devolving powers to the assembly in Belfast to allow them to keep similar customs arrangements to Ireland on agriculture and energy.
But the pro-British Democratic Unionist Party (DUP), the largest in Northern Ireland, reacted angrily to any suggestion of creating separate rules for the province.
It warned that agreement on those terms would threaten its support for British Prime Minister Theresa May's Conservatives, which keeps her minority government in power.
"If there is any hint that in order to placate Dublin and the EU, they're prepared to have Northern Ireland treated differently than the rest of the UK, then they can't rely on our vote," DUP lawmaker Sammy Wilson told the BBC.
Former Northern Ireland first minister Peter Robinson, a former leader of the DUP, called on Dublin late Thursday to stop interfering, saying: "In layman's terms, the South needs to wind its neck in."
'Speculative' border proposals
The British government says negotiations are continuing with Ireland, but its proposals for a solution were savaged on Friday by a committee of MPs, who warned that a "hard border" in Ireland seemed inevitable.
London has suggested a new "highly streamed" customs arrangement using technology to reduce border customs checks, or a new customs partnership with the EU.
"Our report concludes that we cannot at present see how leaving the customs union and the single market can be reconciled with there being no border or infrastructure," said Brexit committee chairman Hilary Benn.
"Even by their own admission, the government's proposals are untested and speculative."
EU leaders meeting in Brussels on December 14 and 15 will decide if there has been "sufficient progress" on the Irish border, Britain's financial settlement and EU citizens' rights to move to talks on to trade.
A deal is close on the latter issues, but failure to make headway on Ireland would deal a major blow to Britain's hopes of agreeing a new trade deal with Brussels before it leaves the EU in March 2019.
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![What's causing the rain and flood in Dubai? [AP]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "What's causing the rain and flood in Dubai? [AP]")
