This article argues that the recognition of sovereignty over Northern Ireland, internationally, and within Ireland, has shifted in the aftermath of the 2016 Brexit referendum. The framework that governs this relationship between Ireland, the UK and Northern Ireland was redefined with the signing of the Good Friday Agreement (GFA) in 1998. In the altered political circumstances of the Brexit negotiations, this redefinition has produced unanticipated consequences. First, it underpinned the high level of support given to the Irish government and to the provisions of the GFA by the EU as an institution, and by EU member states, manifested in the refusal of the EU to negotiate a land border on the island of Ireland. For the UK this was an unforeseen outcome as its negotiation strategy was based on the EU prioritising the importance of accessing the UK economy over Irish claims under the GFA. Second, the undermining of the political stability and relative consensus created by the GFA has led to a new discourse on Irish unity across the island of Ireland, including on the potential shape of a new Ireland. This is visible in the mainstream media, on social media and in the findings of opinion polls. Whether or not these changes will lead to a united Ireland in the short term is uncertain, but the manner in which the sovereignty of the UK over Northern Ireland is recognised has already undergone a fundamental shift, internationally and within Ireland.