Ryanair has had operated a policy of COMPULSORY online check-in for quite some time now. YOU CAN NOT CHECK-IN AT THE AIRPORT.
Check-in for any flight commences 15 days before that flight, and ends 4 hours before departure. So, if you're away for a week or a fortnight you can check-in, in both directions, before departure. Otherwise you'll have to find an internet café (or similar facilities), while abroad, to check in for the return flight.
The check-in fee is £6 (or €6 for flights originating outside of the UK). However many tickets purchased with Ryanair are promotional offers, where the fee is waived. (I've never had to pay it). You MUST print out the confirmation you're provided with and you MUST present it at the airport. (If you don't, Ryanair will charge you £40 or €40, as appropriate, to print it out at the airport).
On arrival at the airport you simply take your bags to the bag drop desk and then proceed through security.
Chris