Sorry. Your information is out of date.
If
(a) your 10kg bag doesn't exceed 55cm x 40cm x 20cm ; AND
(b) there's still enough space left in the overhead lockers when you book ;
then you can pay between £6 to £20 (depending upon the route and date) in advance for 'Priority & 2 Cabin Bags'. You can then take your large bag (together with the regular small one, which you're allowed anyway) into the cabin. The option to do so, subject to space, is available up to 2 hours before departure (or up to 40 minutes before departure if you use the Ryanair app).
If there's no space left when you try to book the above service, or if you're beyond the time limits stated, you can pay between £10 and £25 (again, depending upon the route and date) to have your bag (which must conform to the size limits above) placed in the hold.
If you simply turn up at the gate without having followed either of the above procedures, your bag can still be placed in the hold but the maximum fee (of £25) will automatically be applied.
https://www.ryanair.com/gb/en/useful-info/baggage/bag-policy