If Aer Lingus does not let you check in online, there are specific reasons based on airline policies and passenger requirements. Aer Lingus online check-in typically opens 48 hours before departure for most flights and closes 2 hours before short-haul flights and 2–3 hours before long-haul flights. However, not all passengers are eligible for online check-in. The most common reason Aer Lingus blocks online check-in is travel document verification. If you're flying to or from countries that require strict passport or visa checks, you may be asked to check in at the airport so airline staff can confirm your documents in person. Another common reason is if you booked your ticket through a third-party travel agency and the booking has missing or incorrect passenger information. In this case, online check-in will be restricted until the issue is fixed. Aer Lingus may also deny online check-in if your itinerary includes flights operated by partner or codeshare airlines, as these segments often require manual processing or separate check-in through the partner airline. If you're traveling with special service requests, such as unaccompanied minors, medical assistance, or pets in the cabin or hold, online check-in is usually disabled to ensure proper handling at the airport. Similarly, group bookings with more than nine passengers may not be eligible for online check-in and must use airport counters. Technical errors can also prevent online check-in. Website or mobile app issues, outdated browsers, or poor internet connections might disrupt the process. In these cases, clearing browser cache, updating the app, or switching devices may help. If the system still fails, you’ll need to check in at the airport. Additionally, if you have an outstanding payment, such as unpaid baggage fees, seat selection charges, or other optional services, the system may not allow you to proceed with check-in until those fees are cleared. It’s important to verify payment confirmation before attempting check-in. In some cases, if your flight was recently changed or rebooked, online check-in access might not be immediately available. Always double-check your updated itinerary and use the new booking reference if one was issued. Certain security regulations or airport-specific restrictions may also require passengers to check in at the counter. For example, flights from specific international airports might not support mobile or web check-in due to local rules. If online check-in is unavailable, arrive at the airport at least 3 hours early for international flights or 2 hours early for European routes to avoid missing check-in closure times. In summary, Aer Lingus may not let you check in online due to document requirements, booking issues, special services, codeshare flights, unpaid fees, or technical problems. Always review your booking, ensure all data is correct, and confirm your eligibility before trying again. If you still face issues, contact Aer Lingus customer service or proceed to the airport check-in desk for manual assistance.