Offering an agreed and democratic path back to the EU, Brexit puts the reunification of Ireland centre stage.

There is a need for careful planning and preparation from the Irish and British governments and there is nothing to prevent such work commencing.

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In our view, however, this must also include the EU, and its potential role has so far been neglected; a gap that is addressed by our report The EU and Irish Unity: Planning and Preparing for Constitutional Change in Ireland, launched this week at the European Parliament.

French President Emmanuel Macron and Taoiseach Leo Varadkar have recently noted what has been apparent for more than 20 years: the Good Friday Agreement contemplates Irish reunification following referendums in both jurisdictions on the island.

The fundamental significance of this right to self-determination - overwhelmingly endorsed by the people of Ireland on 22 May 1998 - is widely acknowledged.

In a valuable contribution in April 2017, the 27 EU leaders stated that: "… the Good Friday Agreement expressly provides for an agreed mechanism whereby a united Ireland may be brought about through peaceful and democratic means; and 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."

The EU views the reunification of Ireland as a reshaping of the borders of the state and it is likely to follow the German reunification precedent of 1990, subject to some qualifications.

German unification 'model'

Critically, however, neither treaty amendment nor the accession of a new reunited Ireland will be necessary. What emerges may well involve new constitutional arrangements but it will not be a "new state".

Europe's response will also be guided by its commitment to democracy, human rights, international law and solidarity among member states.

The choice to effect unity or continue partition is one for the people of the island of Ireland alone.

Should Brexit occur, there will exist a body of EU citizens outside the territory of the Member States who will have been removed from the EU despite their expressed wish to remain, deprived of many of the tangible benefits of membership. In our view, the recognition of their right to re-join the Union, through Irish reunification, should be at the forefront of the EU's approach.

This would be consistent with international law and with the EU's own comparative experience and practice.

Vital preparatory work is required and the EU should make a start in expectation of unity referendums.

The practical consequences will depend on the precise nature of the future relationship with the UK, but there are matters that can be anticipated and must be dealt with in advance.

These issues include the confirmation by all EU institutions of the approach adopted by the European Council in April 2017 and the consideration of representation for the north of Ireland in the European parliament.

The impact on Ireland of the current economic and monetary union rules in the event of reunification and what derogations or transitional measures would be necessary should also be on the agenda alongside what amendments to EU and Irish law are required to safeguard, for example, the position of British citizens resident on the island of Ireland.

Following Brexit the EU and UK will negotiate the future relationship.

The right to self-determination, as provided for in the agreement, must feature prominently in these discussions. Recall again, the north of Ireland has, as a matter of right, a possible way back to the EU.

Such an approach is consistent with the EU's commitment to democracy, human rights and the rule of law in its external trading relations, as well as its endorsement of the Agreement in all its parts, and with the UK's obligations in international law.

Clauses relating to human rights are, for example, also present in the EU's recent free trade and accession agreements.

In our view, reassurance will be essential as Ireland moves towards a collective decision on new constitutional arrangements.

EU assistance

As indicated, the EU must, for example, consider the implications of the equality of citizenship and parity of esteem provisions of the agreement and, along with the Irish government, determine what further guarantees should be provided.

The north of Ireland has a way to return to the EU that is central to the constitutional compromise at the heart of the Good Friday Agreement.

The EU must now play its part in providing clarity and certainty to the people of Ireland as they face into referendums that will determine their constitutional future.