I have actually read the Good Friday (Belfast really) Agreement
It's not that long - 35 generously spaced pages.
The word "border" is used ten times. Eight times as "all-island and cross-border". One mention of "cross-border workers" and one mention of the need for emergency services to operate cross-border. This would seem to accept that there will indeed be a border.
It does state that there should be "as early a return as possible to normal security arrangements" but what could be more normal than a border post between two countries?
On the other hand there is a crystal-clear statement that "We reaffirm our total and absolute .... opposition to any use or threat of force by others for any political purpose, whether in regard to this agreement or otherwise."
So how come a customs post is a regarded as a clear violation of the Agreement yet saying darkly "The men of violence won't put up with a hard border" is ok?