EU leaders are expected at a summit this weekend to back automatic membership for Northern Ireland after Brexit if it ever reunifies with Ireland, sources said on Friday.
Ireland will ask the 27 EU leaders to endorse a text on the issue when they meet on Saturday without Britain to adopt guidelines for Brexit negotiations.
While the move could rile London, European sources said it would only "state the obvious" and compared it to integration of the former East Germany into the EU after the fall of the Berlin Wall.
A draft of the summit minutes seen by the reporters says the 1998 Good Friday peace accord "expressly provides for an agreed mechanism whereby a united Ireland may be brought about through peaceful and democratic means".
"In this regard, the European Council acknowledges that, in accordance with international law, the entire territory of such a united Ireland would thus be part of the EU," it says.
Britain's vote to leave the EU has raised a host of sensitive issues about the future of Northern Ireland, with the EU insisting in the summit guidelines on the need to avoid a "hard border" with Ireland.
The historic Good Friday accord, which ended decades of violence in the British province, includes provisions for Northern Ireland to reunify with the Republic in the future if it chooses to.
Britain is already highly sensitive to territorial issues in the Brexit talks after the guidelines said Spain should have the final say over whether any eventual trade deal applies to the British outcrop of Gibraltar.
The issue of a united Ireland rejoining the EU will not however be included in the summit guidelines themselves, an EU Council source said.
Brussels insisted that if added on Saturday a so-called "Kenny text", named after Irish Prime Minister Enda Kenny, would not change the situation of Ireland or Northern Ireland.
By contrast, the EU has said that Scotland would have to reapply to join the bloc if it voted for independence from Britain after Brexit.
The EU says questions about Northern Ireland are among those that must be resolved as part of the Brexit divorce agreement before talks can begin on a future EU-UK trade deal.
The other key divorce issues the EU wants to settle are Britain's exit bill, estimated by EU sources at 60 billion euros, and the fate of three million EU citizens living in Britain, plus a million Britons living in the EU.