European leaders are expected to spell out how Northern Ireland could rejoin the EU after any future border poll.
The leaders are meeting at a summit in Brussels over the weekend to confirm their guidelines for the Brexit negotiations.
Sinn Fein President Gerry Adams said any discussions around the potential for a united Ireland within Europe are to be welcomed.
But the DUP's Ian Paisley said the discussions mean little in reality.
He said: "It falls in to a hypothetical fantasy politics point of view, what would happen if?
"The fact of the matter is, if there was a border poll, people would know that they were voting to leave the United Kingdom and go into a Europe that could not pay for Northern Ireland."
Reports suggest Taoiseach (Irish Prime Minister) Enda Kenny will ask his EU partners to back the idea of Northern Ireland automatically joining the EU if people vote to unite with the Republic.
Mr Adams said: "One would presume that the European Commission, recognising the international treaty which is the Good Friday Agreement, would also recognise the right of people here to be part of a United Ireland if that's what they vote for in a referendum on Irish unity.
"The question is this, will that be a note in the minutes, or will that be a directive as part of these guidelines.
"This is what happened with East Germany, it's a very modest, simple, but very welcome step in the right direction, to recognise if there is to be a united Ireland the entire island would be within the European Union."
UK government view
The Brexit Secretary David Davis said in March that the government's view on Northern Ireland's status within the European Union, in the event of a future border poll in favour of a united Ireland was that should the people of Northern Ireland vote to leave the UK, they would "be in a position of becoming part of an existing EU member state, rather than seeking to join the EU as a new independent state".
Mr Davis made the remarks in a letter to the SDLP MP Mark Durkan.
The EU issued draft guidelines on Brexit on 31 March.
Official talks will not begin until after the UK general election on 8 June.