DUBLIN (Reuters) - The key to British hopes of moving on to talks on trade ties with the European Union after it quits the bloc lies in Dublin, the European Council president said on Friday as Ireland called for  credible  solutions from London for the Irish border.  Avoiding a return to a  hard border  on the island of Ireland is the last major hurdle before Brexit talks can advance to trade ties and a possible two-year Brexit transition deal. But European Council President Donald Tusk that while the EU negotiating team represents the interests of the 27 other members, Ireland will have the final say on the border issue.   Let me say very clearly: if the UK offer is unacceptable for Ireland, it will also be unacceptable for the EU,  Tusk said in Dublin at a joint press conference with Irish Prime Minister Leo Varadkar.   The key to the UK s future lies - in some ways - in Dublin, at least as long as Brexit negotiations continue.    Tusk confirmed he had given British Prime Minister Theresa May a deadline of Monday to make a final offer on Irish border conditions before EU heads of government decide whether there is  sufficient progress  on a UK-EU divorce settlement to merit opening talks on the future relationship. If London meets the three key EU conditions on its financial bill for leaving, the rights of expatriate citizens and the border, then leaders could give a green light to trade talks at the summit on Dec. 14-15. With significant headway on the financial settlement and citizen rights now apparently in the bag, a deal on the Irish border appears to be the final hurdle. The political and economic stakes are high. The economy of Ireland, north and south, has become deeply integrated since the EU single market s creation in 1993, and only road signs now mark the frontier. Free-flowing commerce, together with the 1998 peace deal between Northern Ireland s Protestants and Catholics, has transformed previously neglected areas on both sides of the boundary. Varadkar said that while progress on the border issue had been made, the next few days would be crucial and that Ireland would not be afraid to use its veto if necessary.   The UK must offer credible, concrete and workable solutions that guarantee that there will be no hard border...whatever the future relationship between the EU and the UK is,  he said.  I m an optimist by nature...However I m also prepared to stand firm with our partners if need be...if the UK offer falls short.   Before it can sign off on the first phase, Dublin wants May to spell out in writing how she intends to make good on a commitment to avoid turning the clock back to a border of customs and security checks. It says the best way to do so is to keep regulations the same on both sides of a border that will be the UK s only land boundary with the EU after Brexit. May s government has said Britain will leave the EU s single market and customs union but wants the Irish border to remain open, a stance that EU officials say is contradictory. Earlier on Friday Irish Foreign Minister Simon Coveney said a deal on the border was  doable  but that the sides were  not there yet. Coveney said negotiators were working to find  sensible wording , drafts of which were being exchanged. Dublin, he said, will insist  there will be no fudge . May s room to offer additional concessions to Dublin appeared extremely limited when the pro-Brexit Northern Ireland party propping up her government on Thursday hinted it might withdraw its support if she gives up too much.  But Coveney said the Irish government and the Democratic Unionist Party (DUP) agreed on far more than it disagreed on.  May is also under pressure from British business leaders who want confirmation of a transition deal so they have time to adapt to the new relationship.  