This is from the t&cs from Ryanair ...

To ensure compliance with Visa and Immigration requirements, non EU/EEA citizens must go to the Ryanair Visa/Immigration Desk at the airport to have their travel document approved for travel and their boarding pass validated prior to entering the airport security and screening area.

Not sure whether this applies to you, but it sounds like it needs to be done prior to the gate. I wonder if one-way flights to international destinations might have a similar requirement.