The EU's chief negotiator has emphasised that "a lot more substantial work" is required to preserve Irish cross-border co-operation after Brexit.
Michel Barnier was speaking after a meeting with Irish Foreign Minister Simon Coveney.
At negotiations last week the UK and EU made good progress on maintaining the UK-Ireland common travel area after Brexit.
The arrangement between the UK and Ireland pre-dates the EU's formation.
It allows passport-free travel between the UK and Ireland and gives British and Irish citizens a range of rights in each other's countries.
However it also emerged that the totality of cross-border institutions and relationships has not yet been defined nor has there been a substantial analysis of how Brexit could impact on them.
To that end the UK and EU have agreed a joint "scoping exercise", being described as a substantial piece of technical work which will take months rather than weeks to complete.
Mr Coveney said Ireland is in "a uniquely vulnerable position" because of Brexit and faces potentially "extraordinarly negative" impacts.
He said Ireland was in weekly "if not daily" contact with the European Commission about Irish Brexit issues.