The issue of a border between the north and south of Ireland can be as simple or as complex as the participating sides want it to be. Border controls have had no uniformity for a considerable time. Norway is in the Schengen area but not in the EU and shares a long border with Sweden(EU member). The UK is in the EU but not part of the Schengen area.
Is the purpose of border controls to control the passage of people, or goods, or both? In recent years the supposed threat of terrorism has been used to suspend/alter the schengen agreement for various lengths of time. To my mind having passport control at an airport is just to irritate people. The passenger manifests have a complete passenger profile before departure. Anyone flagged could be easily met at the gate, prior to the remaining passengers deplaning.