The question of the Irish land border was the most problematic aspect of the negotiations on the United Kingdom’s (UK) withdrawal from the European Union (EU). The Irish border aspects of the Brexit negotiations have demonstrated that the border and the maintenance of the Good Friday Agreement is not just an issue for British-Irish relations, but one that now has a strong EU dimension. This article analyses the political impact of alternative proposals tabled during the Brexit negotiations on Northern Ireland and the question of the Irish Border. It places this discussion in the post-conflict context and in the highly politicised nature of the Brexit referendum debate in Northern Ireland. It examines how the issue was framed, following a tortuous negotiation process, in the draft Withdrawal Agreement of 2018 and the ultimate failure of the UK government to ratify that agreement in Parliament. It evaluates the political impacts of the crisis in British politics caused by Brexit and the way in which Brexit has undermined the political stability created by the Good Friday Agreement and at the same time changed the discourse on Irish unity. It argues that failure of the British Government to accurately assess the EU27 position is at the heart of their failure to negotiate a Withdrawal Agreement, for which they could build UK parliamentary support. It is this failure of political judgement that has led to the rejection of the negotiated Withdrawal Agreement and continued to block agreement on a way forward, in the period prior to the October 2019 deadline.