The English channel and customs controls do not interfere with the right of EU/UK nationals to move freely across the border. If hard borders are not instigated on the NI/ROI border, then there is no physical barrier to prevent free movement.

Do you actually read, and think about, what you post.
There may be no physical border between NI/ROI, but we have a freedom of movement arrangement between us anyway. There are physical barriers between UK/(rest of)EU, and between ROI/(rest of)EU.