Ignore the 20cm part of the Ryanair cabin allowance. The Ryanair sizer is open at the front so unless you have a bag that is over around 23cm it will be fine. The height and width of the bag are important though as these parts of the sizer are fixed there is maybe 1cm leeway on the height.
I have a the Away Carry On case which lists the dimensions as 55.1 cm x 36.7 cm x 22.9 cm. It fits like a glove in the Ryanair sizer. Note that the Away Bigger Carry On will not fit due to the height. The Away Everywhere bag also fits perfectly in the personal item sizer. I've also travelled with the Aluminium Tumi 19 Degrees Continental Carry-On 56 cm and I've tested it in the Ryanair sizer out of curiosity and it does fit although it's quite heavy so may not comply with the 10kg rule if they actually bother weighing hand luggage (in my limited amount of flying on Ryanair I've never seen them weigh hand luggage).