The period for online check-in is actually between 15 days and 4 hours prior to the flight time. Most travellers with Ryanair won't be going away for more than a fortnight, so the 15 day period allows them to check-in (in both directions) on the day before their outbound flight.
As Redhelen indicates, on arrival at the airport you simply take your bags to the bag drop desk for tagging and then proceed to the security gates. The tagging process is fairly quick but, for the most popular flights, there can still be some fairly big queues.
Chris