Some of the answers on this thread aren't up to date.
Firstly, online check-in used to require you to go online two days (or less) prior to each flight. Unless you had access to the internet while abroad that made using online check-in for your return flight impossible. That was changed last year.
You can now check-in online 15 days prior to departure (and up to 4 hours before the flight).
Secondly, online check-in used to be restricted solely to customers only taking hand baggage.
Since 19/03/09 online check-in is now available to almost all passengers, including those with checked baggage. (On arrival at the airport you have to take your cabin baggage to a bag drop desk but there are no passport or other ID checks at that point)
http://www.ryanair.com/site/EN/faqs.php?sect=c hk&quest=whoco
Chris