The Ryanair luggage policy helps shred some light on the topic:
It seems that the allowed dimensions for the 10kg check-in bag are the same as those for priority 2-bag carry-on fare, i.e 40cm x 20cm x 55cm. I am inferring this from the sentence (emphasis mine):
Non-priority customers who wish to bring a second larger wheelie bag
where the phrase "larger wheelie bag" refers to the extra bag that priority customers can bring as carry-on. Indeed, the maximum allowed carry-on size for non-priority customers is 40cm x 20cm x 25cm. However, priority customers can also bring a larger 10-kg bag with dimensions 40cm x 20cm x 55cm.
To be safe in your situation, you should probably purchase the 20kg check-in baggage allowance. You can try purchasing the 10kg check-in option and hope that the bag will not be measured, but I do not know exactly how the check-in procedure of these bags work.