While for many the issue of Irish reunification has never gone away, it has acquired increased salience recently in the context of Brexit. In the 2016 EU referendum, 58% of voters in Northern Ireland voted for Remain. This result, and the continuing uncertainty over the impact of Brexit on the Irish border, has led to calls for a border poll to allow the whole island of Ireland to remain in the EU.
A border poll is the term for a referendum on Irish reunification. This would take place simultaneously in both the Republic of Ireland and Northern Ireland.
The only previous time a referendum on reunification was held was in Northern Ireland alone, in 1973. The official result was a vote of 99% in favour of remaining in the UK. However, this vote was boycotted by the Irish nationalist community.
In April, Sinn Féin’s leader in Northern Ireland, Michelle O’Neill, called for this to happen within five years. The nationalist Social and Democratic Labour Party (SDLP) support a referendum on Irish reunification after the Brexit negotiations, though their Deputy Leader Nichola Mallon has warned that a rushed poll could "erupt in violence".
All unionist parties in Northern Ireland oppose a vote on reunification. The Democratic Unionist Party (DUP) recently expressed the wish to put a border poll off "for generations".
In May, the Secretary of State for Northern Ireland Karen Bradley said that the conditions for a referendum had not been met. This followed reports that Theresa May doubted whether the unionist position would win in a vote on Irish reunification.
The Irish Government has also taken the position that the time is not right for a border poll. In July 2017, Irish Taoiseach Leo Varadkar stated that a vote on Irish reunification at this point would be defeated, and therefore "it would not achieve a united Ireland, but what it would do is give rise to further nationalist, further sectarianism and further polarisation."
Data from the Northern Ireland Life and Times Survey has consistently shown that Irish reunification is supported by no more than a quarter of voters. A majority favour a resumption of devolution.
The UK Government has the power to call a referendum in Northern Ireland.
The Good Friday Agreement states that "the Secretary of State" should call a referendum "‘if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland."
The Secretary of State would do this by laying before Parliament an Order in Council, specifying the details of the poll and the date on which it will be held.
In the Republic of Ireland, reunification would require a constitutional amendment. This would require legislation to be passed by both chambers of Parliament, before it was put to the people in a referendum.
In the event that either part of Ireland voted against reunification, another poll cannot be held within seven years.
The referendums in the two parts of Ireland would not have the same question on the ballot paper. In the Republic of Ireland, the question would be a vote in favour or against a specific constitutional amendment to bring about Irish reunification. Similarly, in 1998, the Irish people gave their backing to the Good Friday Agreement by approving the 19th Amendment to the Constitution.
In Northern Ireland, the terms of the referendum question would be set by the Secretary of State. The UK Electoral Commission would be expected to test the referendum question to ensure clarity and absence of bias.
In the Northern Ireland referendum in 1998 on the Good Friday Agreement, the electoral franchise used was the same as for Westminster elections. If the same rules applied in a reunification poll, a voter would have to be 18 or over, and a British, Irish or Commonwealth citizen registered in a Northern Irish constituency.
However, as the Secretary of State can specify who is eligible to vote, a different franchise could be adopted. There might, for instance, be suggestions that the voting age should be set at 16, as was the case for the Scottish independence referendum in 2014.
In the Republic of Ireland, the franchise for referendums is restricted to Irish citizens on the Register of Electors. Registered Irish voters remain eligible to vote for 18 months after leaving Ireland.
As a result, British citizens in Ireland could not vote in a border poll, whereas Irish citizens resident in Northern Ireland could do so.
Based on the current rules, in neither referendum would EU citizens be eligible to vote.
The Good Friday Agreement committed the UK and Irish governments to respect the ‘principle of consent’.
This states that the two parts of Ireland have the right to be reunited, so long as that decision is "achieved and exercised with and subject to the agreement and consent of a majority of the people of Northern Ireland."
If both Northern Ireland and the Republic of Ireland voted in favour of reunification, the Good Friday Agreement further specifies that "it will be a binding obligation on both Governments to introduce and support in their respective Parliaments legislation to give effect to that wish."
The precise terms that Irish reunification would take would have to be worked out between the UK and Irish governments. One important question would be whether separate arrangements were retained for Northern Ireland, or whether the six counties of the North would be fully integrated into the unitary Irish state.
In August 2017, a joint-committee of the Irish Parliament suggested that a reunified Ireland could maintain aspects of the Good Friday Agreement after reunification. For instance, it suggested the Northern Ireland Assembly could be continued as a devolved regional parliament within Ireland.
It further suggested that the intergovernmental British-Irish Council could continue, allowing for an ongoing British role in the matters of Northern Ireland to reassure the unionist community.
These recommendations had cross-party support in the committee and built on proposals previously made by the nationalist SDLP party in Northern Ireland.