"If in London someone assumes that the negotiations will deal with other issues first, before moving to the Irish issue, my response would be: Ireland first," Tusk said after talks with Irish Taoiseach Leo Varadkar in Dublin.
Both Britain and the EU have vowed to avoid the return of customs checks to the border between Northern Ireland, which is part of Britain, and EU member Ireland after Brexit.
They reached an interim deal in December leaving some flexibility on the issue, but an EU text putting the agreement into law has sparked a fresh row with London.
Britain intends to leave the EU's single market and customs union, which suggests the need for border controls somewhere.
The EU text says Northern Ireland must stay in a customs union with the rest of the bloc if no better way is found to avoid a hard Irish border -- which Britain rejects.
"While we must respect this position, we also expect the UK to propose a specific and realistic solution to avoid a hard border," Tusk said in Dublin.
"As long as the UK doesn't present such a solution, it is very difficult to imagine substantive progress in Brexit negotiations."
Varadkar said: "We must have certainty that, if a better option proves unachievable, the backstop of maintaining full alignment in Northern Ireland will apply."
Britain hopes to begin talks on the future trading relationship with Brussels next month, and Prime Minister Theresa May set out her proposals for a new wide-ranging free trade agreement (FTA) in a speech on Friday.
Finance Minister Philip Hammond set out more detail this week, saying it would be in both sides' "mutual interest" to include the financial services industry in the deal.
The EU has repeatedly rejected this, although its draft guidelines for the trade talks published on Wednesday made no overt mention of the issue.
Tusk repeated Thursday that a free trade deal could reduce or eliminate tariffs on goods, but services required common rules to ensure there was a level playing field.
"This is why we cannot offer the same in services as we can offer in goods. And it's also why FTAs don't have detailed rules for financial services," he said.
"We should all be clear that also when it comes to financial services, life will be different after Brexit."
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