You should go to ryanair.com, click on "manage my booking" and add the bag there. If:
* you check in online without adding the bag first, you will have to pay the airport luggage fee which is much higher
* you decide to check in at the airport without adding the bag first, you will have to pay the airport luggage fee and on top of that, a per-person check-in fee of €40. This is the most expensive option one could imagine.
The correct way is to add the luggage via Ryanair's website before checking in, then check in online and drop off your bag at the airport.