Unlike almost all other treaties about borders, in the Good Friday Agreement Ireland does not recognize Northern Ireland as a perpetual part of the UK and does not recognize unconditional sovereignty of UK over NI - it's a recognition of the current situation as reflecting the current wishes of the population but explicitly leaves the long-term outcome open ended, stipulates that UK sovereignty is conditional upon NI population wishing to stay part of UK and mandates that if that choice of NI people changes, then UK has a binding obligation to cede Northern Ireland, no matter what people in England/Scotland/Wales would vote for.