Adding your passport details to a Ryanair booking is a critical step that must be completed during the "Check-In" process, rather than just at the time of booking. To do this, you need to log into your account on the Ryanair website or mobile app and navigate to the "My Bookings" section. Once you select your upcoming flight, click on the "Check-In" button. During the check-in flow, the system will prompt you for Advanced Passenger Information (API). This is where you must accurately enter your document type (Passport or National ID), the document number, the country of issue, and the expiration date. It is essential to double-check these details, as Ryanair is known for being quite strict; any discrepancy between your boarding pass and your physical passport at the gate could result in you being denied boarding or charged a significant fee for a document reissue at the airport. You can typically update this information up until the check-in window closes, which is usually two hours before the scheduled departure time.