The return of passport controls along the border with Northern Ireland is "abhorrent" - but would be a potential consequence of Britain leaving the European Union, Tánaiste Joan Burton has warned.
The Labour leader has also warned that Britain's renegotiation of EU membership could undermine workers' rights as guaranteed by Brussels' social laws.
Ms Burton's comments, to a conference in Dublin of a major British union, GMB, are her first major statements on Britain's 'In-Out' referendum on EU membership expected to take place next year.
The Tánaiste said Ireland respected that British Prime Minister David Cameron had secured the mandate to hold the EU membership referendum and the Government respected the need not to interfere in another country's internal affairs.
But she said Ireland had a big stake in the outcome and Britain's EU membership was important to this country and the rest of the EU.
Ms Burton said the 1998 Good Friday Agreement and the 1992 EU border-free single market had facilitated the free movement of people and goods across the border with the North. These ended the identity controls and customs checks along the border.
"The idea of demanding passports at the border is abhorrent to me and I suspect it would be unacceptable to any Irish Government. We value the ease of movement and the improvement in relations which we have seen in the last few years and we would be loath to give it up," Ms Burton told British union delegates at the Citywest conference centre.
The Tánaiste said Ireland was ready to support Britain's renegotiations and there were potential benefits for all member states on redefining relations within the EU. But she stressed that Britain's approach and aims must be "reasonable".
Ms Burton pointed to British union leaders' warnings that Britain's renegotiations with Brussels and the other member governments cannot "hollow out" workers' rights. She said she wanted to see an EU that was good for business - but it must also uphold social values.
The Tánaiste said EU freedom of movement was extremely important but could not be unlimited and had to take account of the larger concerns about migration across Europe.