Brexit was a UK choice that raised existential challenges for the Irish government. The 1998 Good Friday (Belfast) Agreement created a North–South Ministerial Council to facilitate and intensify cooperation across the land border between Ireland and Northern Ireland. These provisions formed part of a larger compromise that accompanied the peace process and led to a normalization of life in the communities on both sides of the border. The peace accord in combination with EU membership of the UK and Ireland made the border increasingly invisible. Brexit was the proverbial bull in a china shop. It destroyed a fragile political equilibrium.

The UK was about to take back control of its borders. Its decision to leave the single market and customs union risked putting Ireland in an impossible position, as it had to make sure all goods imported from the UK respected EU rules. Because of Brexit, this logically implied controlling the 499 km land border that meanders across the island, a scenario that raised horrific technical and regulatory challenges. Ireland would have to administer entry and exit declarations of all goods crossing that border, possibly charge tariffs and police VAT fraud. It would have to make sure no prohibited or counterfeited goods entered the EU's territory via the land border. Customs issues were just one aspect. Irish authorities would also become responsible for ensuring all goods, food products and live animals entering Ireland complied with EU health and safety standards.

The principle that each national customs authority must play by collective EU rules enables the clearance of goods and their free circulation in the EU. Some pro-Brexit commentators blamed EU bureaucracy for obstinacy and claimed Ireland should dispense with all of this, omitting to add that every sovereign jurisdiction in the world applies similar rules to protect the integrity of its national territory and state revenues. If Ireland could not guarantee the fulfilment of its responsibility as an EU member at the external border, its place in the single market and customs union would be endangered, which could ultimately threaten the survival of that single market in case continental countries started imposing intra-EU border checks on goods coming from Ireland.