You need to go to the Ryanair website ("Manage my booking" part), then click on "Add advance passenger information" and input your passport data and whatever else they ask there (as far as I can remember it is all about your passport: number, when issued, when expires, etc)
There used to be time requirement (I think it was at least 24hrs before your flight) for doing this. In any event, they might refuse you boarding if you haven't done it. So better get it out of your way and enjoy your trip!
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