Unlike other red lines from the UK governments, the Good Friday Agreement is also an international treaty between the governments of United Kingdom and the Republic of Ireland, two sovereign countries. The treaty included many interesting things, like the removal of any territorial claims from the UK to the totality of Ireland and from the Republic to NI, the disarming of paramilitaries: essentially, the goal was to bring an end to the only remaining civil war within the EU.

The main effect of the GFA has been to remove pretty much all checks at the Irish border. A no-deal Brexit could change that however, but the Brexiters don't seem to have considered any effect on Ireland, North and South.