I am not sure exactly what to say under “union protection.” What concrete reasons did the DUP give to oppose the agreement and are these reasons still valid? In a major compromise, the parties agreed on measures to promote the Irish language, which trade unionists have long opposed to the fear that it will increase nationalist and republican culture to the detriment of their own. In return, the agreement contained provisions to promote Ulster-Scots, traditionally spoken by descendants of Protestants from Scotland to Northern Ireland. Negotiations were also reinforced by commitments in Dublin and London for increased funding for hospitals, schools and other social services in Northern Ireland. The agreement came after many years of complex discussions, proposals and compromises. A lot of people have made a great contribution. Tony Blair and Bertie Ahern were the leaders of the United Kingdom and the Republic of Ireland at the time. The presidency was chaired by U.S. Special Envoy George Mitchell.  The final withdrawal agreement between the EU and the UK has opted for a complex solution to this delicate problem.
Under the proposed regime, Northern Ireland, like the rest of the UK, would leave the EU customs union, the basis for common tariffs on all products imported into the bloc. However, the necessary customs checks would not take place at the border with the Republic of Ireland, but between Northern Ireland and Great Britain, creating a new border in the Irish Sea. Meanwhile, Northern Ireland – but not the rest of the UK – would continue to follow many of the EU`s internal market rules, so that the land border with Ireland could remain open. This regime is also supported by a separate agreement between Ireland and the United Kingdom allowing the free movement of persons between the two countries. Northern Ireland political parties that approved the agreement were also invited to consider the creation of an independent advisory forum, which would represent civil society, with members with expertise on social, cultural, economic and other issues, and would be appointed by both administrations. In 2002, a framework structure was agreed for the North-South Advisory Forum, and in 2006 the Northern Ireland Executive agreed to support its implementation. Issues of sovereignty, civil and cultural rights, dismantling of arms, demilitarization, justice and police were at the heart of the agreement. In addition to the number of signatories[note 1], Stefan Wolff notes the following similarities and differences between the issues dealt with in the two agreements: The most controversial topic was the Northern Irish border with the Republic of Ireland. The border, heavily militarized during the conflict, has since become essentially invisible, with people and goods crossing freely. This was largely possible because Ireland and the United Kingdom were part of the EU single market, a common set of rules that allow the free movement of goods, services, people and money within the bloc. The Good Friday Agreement (GFA) or the Belfast Agreement (irish: Comhaonté Aoine an Chéasta or Comhaonté Bhéal Feirste; Ulster-Scots: Guid Friday Greeance or Bilfawst Greeance) is a couple of agreements signed on 10 April 1998 that put an end to most of the violence of the Troubles, a political conflict in Northern Ireland that had erupted since the late 1960s.