The subject of the paper is reflections on the consequences of Brexit for the functioning of the border between Ireland and Northern Ireland. The author explains what this border means when the United Kingdom is no longer a member of the European Union, and thus the border may be a barrier to the free movement of people, goods, capital and services. At the same time, it is stressed that the exit of the United Kingdom from the European Union will have a significant impact on the change of the function of this border, which from then on is no longer an internal border of the Union. The aim of the paper is therefore to analyse the consequences of this change, with a simultaneous indication of different models of the UK’s functioning with relation to the European Union and their impact on the British-Irish relations, especially with regard to the various dimensions of the border as a barrier. The article presents various scenarios of solutions that will determine their mutual relations as a result of negotiations between the European Union and Great Britain, especially with regard to access to the single European market, which in turn will be influenced by the border between Ireland and Northern Ireland in the scope of the free movement of people, goods, capital and services. The author used the system analysis method and the comparative method. The author puts forward the thesis that if the negotiations cause a fairly strong loosening of relations between the UK and the European Union, to mitigate the consequences of such a situation for the Irish-British relations, it will be necessary to find and develop bilateral solutions that will facilitate border crossing. The Smart Border 2.0 concept can constitute such a solution.