Complex sentence: EU countries that are not members of the Schengen Agreement are committed to allow lawful entry by citizens of other EU countries, but may exercise border control at their discretion; this typically presents a significant hindrance to persons who are trying to enter those countries illegally.
Split into simpler sentences: EU countries that are not members of the Schengen Agreement are still committed to allow lawful entry by citizens of other EU countries; they may however exercise border control at their discretion. This typically presents a significant hindrance to persons who are trying to enter those countries illegally.