Irish customers on Ryanair's website and mobile app will be required to have a "My Ryanair" customer account to complete their flight bookings from November 1.
The airline said the move is to prevent "screenscraper" websites from mis-selling Ryanair's fares at higher prices.
It also said the move will make it faster and easier for customers to book flights while at the same time it will increase their online security.
From November 1, Irish customers who have not yet registered for "My Ryanair" will have an account automatically generated at the payment stage on the website and app.
There they will be asked to enter an email address and password in order to join "My Ryanair" and complete their booking.
This feature will be rolled out in other markets during November and December.
Ryanair said that over 15 million people have already joined "My Ryanair" throughout Europe. Members receive special offers and "exclusive" early access to sales on the Ryanair site.
Other airlines, including EasyJet and Virgin, have also made it compulsory to have an account in order to book a flight.