The EU's negotiator says the border issue will be "very difficult"
An overwhelming majority of the European Parliament has adopted a resolution officially laying down key principles for its approval of the UK's withdrawal from the union.
MEPs voted 516 votes in favour, 133 against, with 50 abstentions. Any Brexit agreement will need approval of the parliament.
The resolution says citizens' interests must be at the forefront right from the beginning - noting that Irish citizens "will be particularly affected".
MEPs have urged all parties to remain committed to the Northern Ireland peace process and avoid a hard border.
"The special circumstances presented by this situation must therefore be addressed as a matter of priority in the withdrawal agreement", the parliament says.
On the question of resolving the issue of Irish border, the EU Parliament's chief Brexit negotiator Guy Verhofstadt said: "It will be very difficult, and it will be very high on the priority list.
"For the parliament it is an essential point - that's the reason why in the resolution we say: no hard border, respect in all its aspects of the Good Friday and...it is the EU-27 who have to take on board the interests of the Irish Republic".
The resolution warns the UK against any attempt to limit rights linked to freedom of movement before it withdraws from the EU.
It also asks the other 27 member states to examine how to address "the fear of British citizens" that Brexit will lead to the loss of their current EU citizenship rights.
MEPs stressed the importance of securing equal and fair treatment for EU citizens living in the UK and British citizens living in the EU.
The resolution warns against any trade-off between security and the future EU-UK economic relationship, opposes any sort of cherry picking or a piecemeal economic relationship.
MEPs are calling on both sides to act "in good faith and full transparency" to ensure an orderly exit.
The resolution notes that it would be a breach of EU law for the UK to negotiate trade agreements with third countries before it leaves the EU, and warns against Britain engaging in bilateral talks with other EU member states .
It says while the UK will continue to enjoy its rights as a member of the EU until its departure, "it will also have to shoulder its obligations, including financial obligations stemming inter alia from the current long-term EU budget. Such financial commitments could run beyond the date of departure".
Foreign Affairs Minister Charlie Flanagan has welcomed the adoption of the resolution, saying: "The European Parliament will have an important role to play in the forthcoming Brexit process, so I welcome the resounding approval of today’s comprehensive resolution.
"I am greatly encouraged that the resolution contains a very strong acknowledgement of Ireland’s unique concerns on Brexit.
"This position reflects the sound understanding within the parliament of our unique issues, not least in relation to the peace process, the border and the Common Travel Area, and the need to address these issues effectively in the forthcoming negotiations."