You would either have to go through the process or do online chat with FR to find out. You can find their terms on checked bags, including the link to online chat here:

http://www.ryanair.com/en/terms-and-...heckedbaggage/

It does state that charges are higher if you do it online after making the initial booking. These charges are not included in their table of charges which you can find here https://www.ryanair.com/en/fees/ but one would assume it is no higher than the airport/call centre cost and probably somewhere between that and the cost of doing with initial booking.