This week, British Prime Minister Theresa May announced the Withdrawal Agreement, a plan outlining the United Kingdom's legal departure from the European Union (i.e. the process of Brexit). However, aside from extremely controversial provisions, such as the terms of post-Brexit transition period between the UK and the EU, which has led to multiple members of May's cabinet resigning in protest, there is specifically one point of contention which may threaten the very territorial harmony of the UK.
This concerns the relations that the UK's Northern Ireland has with its southwestern neighbor, the Republic of Ireland. Since Republic of Ireland is planning to remain in the EU, if Northern Ireland leaves the EU along with the rest of the UK, it may be forced to implement a "hard border" with its neighbor due to its now different, non-EU, commercial and customs policies.
In order to avoid a large division on the Irish island, which could impel Irish nationalists to resume attacks similar to those of The Troubles period (where hundreds were killed by terrorist attacks carried out by nationalists seeking to unite the whole of the island under Irish rule during much of the latter half of the 20th century), Prime Minister May has decreed that Northern Ireland "will continue to abide by all of the EU's trading rules." However, this will only lead to a new set of pressing concerns for the political parties of Northern Ireland, as they, especially the Democratic Unionist Party (DUP), are now openly opposing May's Brexit agreement because they do not want abide by the "customs backstop" and be in "EU customs territory" since this would lead to the economic isolation of Northern Ireland from the rest of the post-Brexit UK. In order to win back the support of the DUP and, by extension, a vital parliamentary voting bloc needed to pass the Withdrawal Agreement, Theresa May must find a way to ensure the UK citizens of Northern of their long-term economic security.
In order to understand how the political actors of Northern Ireland can be convinced that the Withdrawal Agreement upholds their interests, the implications of the customs backstop must first be considered. If the UK leaves the EU, it could very well slap tariffs on commerce with polities still involved in the EU.
Since Northern Ireland would continue to have EU commerce policies, a "tariff and regulatory border" would severely harm many businesses in Northern Ireland that conduct commerce with other UK territories, since about Northern Ireland currently exports around £15 billion worth of goods to the rest of the UK, which amounts to nearly 20% of all Northern Irish exports. The economic risk is so great that some unionists (people in favor of Northern Ireland being as big a part of the UK as possible) believe that this customs problem could "jettison Northern Ireland as an equal partner for the sake of greater freedom for Great Britain." In other words, the economic well being of many in Northern Ireland could be sacrificed in order to create a satisfying Brexit deal for the rest of the UK, an agreement that Northern Ireland's politicians are naturally outraged about.
Despite such setbacks, May's government is making some strides towards securing the necessary support needed to pass the Withdrawal Agreement, customs backstop and all. After conducting Theresa May personally met with Northern Irish business organizations at Downing Street, some of them, such as the Confederacy of British Industry (CBI), have chosen to throw their support behind May's Brexit deal, largely in order to avoid the prospect of a Brexit without any concrete agreement, which could make it difficult for Northern Irish businesses to obtain substantive investment from both the EU and the UK.
While May's current attempts to "provide the reassurance that I know is so important to [businesses]" seems scant, it appears to be having an impact. The Ulster Farmers' Union, another prominent Northern Irish business group, directly urged the DUP to reverse its current sentiments and back May's deal. Protests from businesses and their organizations may put political pressure on the DUP in their own regional political stronghold and make them more likely to support the withdrawal agreement. However, the situation remains ugly, as the DUP is "critical of business organisations" that support May's Brexit Deal. There is currently no vote scheduled for the Withdrawal agreement, so much remains to be seen as to whether or not Northern Ireland will fare well in the ongoing Brexit international crisis.