UK Prime Minister Theresa May's pledge to maintain a Common Travel Area with Ireland once Britain leaves the EU has been thrown in doubt by senior Government ministers and key figures in Brussels.
In her long-awaited Brexit speech, Mrs May said she would seek to retain free movement between Ireland and Britain once negotiations begin in March.
Mrs May said she did not want to see the return of a hard border between the North and Republic, and said this would be a key demand during talks.
Mrs May also insisted she intended to take Britain entirely out of the EU single market - and warned the UK would not seek a "partial" or "associate" membership of the union.
The Government welcomed Mrs May's commitment to the Common Travel Area with Ireland, but said it was "under no illusions" of the challenges it was facing and conceded a 'hard Brexit' was inevitable.
Privately, Cabinet ministers raised concerns over how Mrs May would convince her EU counterparts to allow Britain to maintain a Common Travel Area with Ireland, but not with other member states.
Last night, a senior Fine Gael minister said Brussels would demand a hard border between the North and South even if the Common Travel Area between Britain and Ireland remains.
"By listening to the Taoiseach and prime minister, I believe the rest of Europe will insist on a hard border and the Border will have to be manned," the minister said.
A Government source said that Taoiseach Enda Kenny emphasised the importance of no return of a hard border and maintaining the integrity of the common travel area with Mrs May before her speech.
The source said the Government was happy Mr Kenny's comments were taken on board by the prime minster when she noted a "special relationship" between the two countries. But it said details of how a hard border could be avoided "have yet to be worked out".
Meanwhile, in Brussels one senior diplomat said the speech by Mrs May was "not very realistic" and signalled very hard and long upcoming negotiations.
"The decision that Britain is totally leaving not just the single market but also the customs union has huge ramifications for trade and the threat of tariffs. This is not necessarily good news for Ireland," the diplomat said.
A second official source warned that the coming two years would be "a rollercoaster ride" with talks likely to wax and wane. Much would depend on whether the UK economy could hold up over the negotiating period.
"These will be very difficult negotiations with all the other 27 EU member states throwing in their contributions based on their national interests," the source said.
The EU Parliament's Brexit negotiator, Guy Verhofstadt, said yesterday it was an "illusion" to think that Britain could enjoy the advantages of the European Union's single market without accepting the obligations that come with it.
"Threatening to turn the UK into a deregulated tax heaven will not only hurt British people - it is a counterproductive negotiating tactic," he said.
"Britain has chosen a hard Brexit. Mrs May's clarity is welcome - but the days of UK cherry-picking and Europe a la cart are over."
EU Council president Donald Tusk said the union was in the middle of a "sad process" and "surrealistic teams", but added that Mrs May's speech was "at least more realistic" than previously.
In a post on Twitter, he said the 27 member states remained united and ready to negotiate once Mrs May formally declared Britain was leaving by triggering Article 50 of the EU treaty.
Yesterday, Mr Tusk spoke to German Chancellor Angela Merkel and French President Francois Hollande. EU leaders are keeping their negotiating powder dry until Mrs May formally launches talks in the coming months. But officials who are preparing for the process noted that Mrs May had not spelled out many downsides for Britain's economy.
"Where is the give for all the take?" asked the Czech Republic's secretary of state for EU affairs, Tomas Prouza.
Mrs May is due to visit Ireland before the end of the month to hold formal discussion with Mr Kenny about Britain's plans to leave the EU.
The Government said it was "very aware of the potential economic opportunities that may arise for Ireland" from Mrs May's planned 'hard Brexit'.
This included shifting investment, business and job creation as well as luring EU agencies currently located in London - including the European Medicines Board and the European Banking Authority.
A Government spokesman said its priorities remained its economic and trading arrangements, the peace process and Border issues.
"In her speech, Mrs May highlighted the specific and historic relationship between Britain and Ireland," a spokesman said.
"In this context, she made clear that her priorities include maintaining the Common Travel Area and avoiding a return to a hard border, both of which are welcome. The alignment between our concerns regarding the economy and trade and the objective of the UK to have a close, and friction-free, economic and trading relationship with the EU, including with Ireland is also very important."
Once Theresa May said she was insisting on immigration controls, rejecting the jurisdiction of the ECJ, and making trade deals with non-European Union countries, a 'hard' Brexit - along the lines of the model in her speech yesterday - became inevitable.
Forget talk of a 'hard' Brexit, from our point of view it's a brutal one. If a week is a long time in politics, then 30 of them must be an eternity - long enough certainly for Theresa May to transform from a Remain supporter (albeit perhaps a lukewarm one) to the advocate of a Brexit that is almost straight from the Ukip handbook.
There was some cheer for Irish business after British prime minister Theresa May's speech, as sterling rocketed to its biggest daily gain since at least 1998 to the benefit of Irish exporters and those competing with British imports.