The airline's website and app is to "close" for five hours from 21.00 (GMT) on Tuesday, August 11 to 02.00 (GMT) this Wednesday, August 12.
The website and app are closing "for a systems upgrade" in preparation for the launch of its new website this autumn, the airline said. This is the second time this summer that the website and app will have been shut down, following a 10-hour closure on June 19 last.
The June shutdown was pre-planned to facilitate maintenance of its online booking system, Independent Travel reported.
The airline's online check-in service will not be available during the closure, and Ryanair is calling on passengers travelling this Tuesday and Wednesday to check-in online for their flight today - i.e. Monday, August 10.
New bookings or changes to existing flights will not be possible during the web closure.
Ryanair is contacting all intending customers travelling this Tuesday and Wednesday by email and SMS text to advise them to check in online in advance to avoid disruption.
The airline apologised for any inconvenience. Ryanair.com has 1.2 million unique visitors a day.