The Irish government supported this proposal.  It was strongly rejected by the Democratic Unionist Party as a weakening of Northern Ireland`s place in the UK and is seen as the main reason why Theresa May`s withdrawal agreement was never approved by the British Parliament.  The British government had rejected the original proposal. At the start of the Brexit talks, Britain and the EU pledged to avoid a hard border between Northern Ireland and the Republic. The UK agreed that if no alternative solution could be found, it would be aligned with EU rules on the internal market and customs union in order to keep the border open and without infrastructure. This obligation has been called backstop. It has become the most controversial topic of the Brexit talks and has led to Theresa May`s deal being rejected three times in the House of Commons. The backstop eventually became a “front stop” in Boris Johnson`s withdrawal deal, with Northern Ireland expected to continue to abide by the rules of the EU`s internal market and customs union, while the rest of the UK could split. This avoided a hard border, but had to carry out checks on goods between Britain and Northern Ireland.
Under the agreement negotiated by Theresa May, after the official exit from the EU, the UK would enter a transition period during which it would remain a member of the body`s economic zones, namely the internal market and customs union. The completion of the European Single Market in 1992 (initiated by Commissioner Lord Cockfield) and the Good Friday Agreement (negotiated by US Senator Mitchell) in 1998 were considered possible to dismantle the important border infrastructure between Northern Ireland and the Republic of Ireland.  In October 2019, the new Johnson government renegotiated the project and replaced the backstop. Under the new protocol, the whole of the UK leaves the EU customs union as a single customs territory. Northern Ireland will be included in the UK`s future trade agreements, but will not have tariffs or restrictions on goods crossing the Irish border in both directions, creating a de facto customs border in the Irish Sea with Britain. There is also a unilateral exit mechanism for the Northern Ireland Assembly to leave the protocol by a simple majority.    This new protocol has been described as “Chequers for Northern Ireland” by some, as it is comparable to the British plan for future relations of Theresa May`s Chequers, previously rejected by the EU and criticised by Johnson.  On 14 November 2018, after a five-hour cabinet meeting, Prime Minister May announced that her cabinet had approved a draft withdrawal agreement with the EU.    On the same day, the Government issued an opinion on the agreement on the withdrawal of the United Kingdom from Great Britain and Northern Ireland from the European Union and stated that negotiations on future relations between the United Kingdom and the EU were under way and that the (binding) withdrawal agreement would not be signed without a (non-binding) political declaration on future relations. , “on the basis that nothing is agreed until everything is agreed.”  After Brexit, the border between Northern Ireland and the Republic of Ireland became an external border of the EU.  In theory, a “hard” border could return, both with fewer and monitored border crossing points, to support the necessary customs infrastructure.
 Both the EU and UK negotiating teams have made it clear that this result will not be acceptable in any final withdrawal agreement.   The Irish backstop is essentially a safety net that would prevent the reintroduction of a hard border on the island of Ireland after Brexit.