The article analyzes Brexit as a factor influencing the Good Friday Agreement and the transformation of the peace process in Northern Ireland, as well as the relationship between Northern Ireland and the Republic of Ireland. It is argued that Brexit poses a threat to the security of Ireland and the EU, since it undermines the foundations of the peace process in Northern Ireland and creates the likelihood that the border will be closed on the island of Ireland again. The Good Friday Agreement of 1998 was based on the fact that both signatory states are members of the EU, and many of its provisions will not be implemented after Brexit. It has been proven that after Brexit, the most controversial issue was the border between Northern Ireland and the Republic of Ireland, which became the external border of the EU. The border, which was heavily militarized during the conflict, became virtually invisible after its settlement, and people and goods freely cross it. This became possible largely because both Ireland and the UK were part of the EU single market, a common set of rules that ensures the free movement of goods, services, people and finances within the bloc.It is argued that the new relationship between the EU and the UK, which was agreed during 2020 and implemented from January 2021, resulted in an attempt to resolve the complex border issue, which was carried out in the Northern Ireland Protocol. The arrangements of the Northern Ireland Protocol, according to which Northern Ireland, but not the rest of the UK, remains within the framework of the EU single market for goods, make it possible to maintain an open border between Northern Ireland and the Republic of Ireland. The establishment of an open border was a key aspect of the Northern Ireland peace process and the 1998 Good Friday Agreement, which ended the conflict in Northern Ireland. It is shown that the Northern Ireland Protocol creates a de facto customs border in the Irish Sea between Northern Ireland and the United Kingdom. Mainly to address the concerns of the DUP unionists about the Northern Ireland Protocol, in 2022-2023, the EU and the UK agreed on revised mechanisms for its operation – the Windsor Framework, which entered into force on March 24, 2023. The DUP refused to accept the Framework Program as meeting their interests until early 2024. Further adjustments to its work have not been agreed upon and the formation of a new Northern Ireland executive has not begun. It is proved that Brexit is becoming a new and significant factor in the unification of Northern Ireland with the Republic of Ireland. Brexit has put the issue of the unification of Ireland at the top of the political agenda of both parts of Ireland. The majority of Northern Irish residents voted to remain in the EU in the 2016 UK referendum. Unification would give Northern Ireland, where both British and Irish citizens live, full EU citizenship.