The Ryanair.com website is being closed for more than 24 hours for an upgrade.
The site will be unavailable from 10pm on Friday until midnight on Saturday.
Passengers have been urged to check-in online before 4pm on Friday, although flights will not be affected.
The budget carrier says the upgraded site will offer three times faster access to fares and hotel rates.
A new SMS alert service is being introduced for when flights are delayed by more than two hours or during periods of flight disruption.
Ryanair is releasing one million seats at fares starting at €10.99 for travel on Mondays, Tuesdays and Wednesdays in June, which will be available on more than 1,000 European routes for booking made before the upgrade begins.
A spokesman said: “As ‘manage my booking’ will be unavailable during this 24 hour website closure, we have advised all passengers due to travel over the coming weekend to check-in on Ryanair.com before 4pm Friday May 18.
“From Sunday, Ryanair passengers will enjoy even faster access to our lowest fares and our guaranteed lowest hotel prices after the Ryanair.com website has been upgraded.
“Our upgraded site will improve performance, and allow us to introduce a new SMS text alert system which will deliver real time info to passengers during flight delays of over 2 hours and during periods of mass disruption, such as adverse weather, air traffic control strikes or airspace closures.”