[dangus]

This, according to the very same BBC:

“What does the Good Friday Agreement say about a hard border?

“A lot less than you might think. The only place in which it alludes to infrastructure at the border is in the section on security.

“During the Troubles there were heavily fortified army barracks, police stations and watchtowers along the border. They were frequently attacked by Republican paramilitaries. Part of the peace deal involved the UK government agreeing to a process of removing those installations in what became known as “demilitarisation”.

“The agreement states that “the development of a peaceful environment... can and should mean a normalisation of security arrangements and practices.” The government committed to “as early a return as possible to normal security arrangements in Northern Ireland, consistent with the level of threat”. That included “the removal of security installations”. That is as far as the text goes.

“There is no explicit commitment to never harden the border, and there is nothing about customs posts or regulatory controls.”

Sounds like no reason a border resembling the pre-9/11 U.S.-Canadian border couldn’t exist. Ireland is an island. You shouldn’t need more of a border beyond customs controls, unlike Ireland is actively undermining border security in ways which would themselves violate the Good Friday accord.

[Winniesboy]

The Good Friday Agreement didn't concern itself with customs and regulatory controls simply because it didn't need to. At the time the Republic of Ireland and the UK (including Northern Ireland) were already in a customs union and single market by virtue of their EU membership. Nobody at the time foresaw that this might ever change, so the issue of customs etc never arose in the lengthy negotiations preceding the agreement. Discussions on the border were concerned entirely with security and freedom of passage, not customs.

[Cronos]

The UK and Ireland both agreed the Good Friday Agreement including its provisions that the border be open, that there be free access between Ireland and NI and that people in NI can be both British and Irish and choose to use either or both citizenships.

The word of the Good Friday Agreement isn’t broken by a hard border but such a border goes against the grain of the stated intents of close cooperation. A principle to which the UK was genuinely committed.

The republic of Ireland isn’t willing to make any compromise on how that border stays open as it see’s the potential for a return to “the Troubles”