Good idea but I've already booked the flight.
I know I can add on luggage before I do the online check in but question is do I have to do it both ways?
I looked at this before and if you do online checkin you have to do it both ways. What you can try and do is do the outward journey and then add in the luggage but I'm not positive you can do it. Ryanair are constantly changing the rules. You can also pay at the airport, the time I did it, it was cheaper to pay at the airport than for return bags online.