Ryanair's website and mobile app will be down for an eight-hour period from 6.30pm tomorrow (Wednesday) to facilitate a systems upgrade.
During this time customers of the airline will be unable to check in for flights online, make new bookings or changes to existing flights.
Customers travelling on Wednesday and Thursday are advised to check in online for their flight before the web closures are in effect.
Affected passengers will be able to check in at their departure airport, however, the airline's airport check-in fee will still apply.
Ryanair said it has contacted all customers booked to travel on Wednesday 22 and Thursday 23 March by email and SMS to advise them to check in online today to avoid the disruption.
Meanwhile, Ryanair also today announced Munich Memmingen as its tenth German base, with one based aircraft at the airport.
A new winter schedule from Memmingen with 17 routes is expected to deliver more than 800,000 passengers annually at the airport.