The only person to perform document checks for domestic segments, e.g. RSW is to prevent the situation where someone is stuck at an international gateway, e.g. NYC because they can't board the international flight. The sole reason carriers check travel documents at all is to protect themselves from liability if a passenger is denied entry on arrival. In theory, Irish authorities could have conducted a passport check at DUB and if your son lacked a passport, denied him entry and required EI to return him to NYC. Customs could have checked his bags at MAN.