The United Kingdom officially left the European Union on 31 January 2020. Almost one hundred years to the day after the Government of Ireland Act 1920 provided for the partition of Ireland, a new border was thus created between Northern Ireland and the Republic of Ireland: an external frontier of the European Union now cuts through the island, and the extent to which its consequences might mirror those of the border established in 1920 is a major consideration for most actors involved in the Brexit negotiations, notably in Dublin and on the European continent.