Please note that Ryanair has two cabin baggage sizes.

1x carry on 1x personal item/bag

If I recall correctly you need to pay extra for the carry on sized baggage. The personal item/bag one is indeed very small and flat. Smaller than what most other airlines allow. This one is supposed to go under the seat in front of you, not the overhead locker.

I have only 2 experiences with Ryanair so far. One more than 10 years ago to a touristic destination. In that flight they were super strict, much stricter than I was used to from other airlines. They made me put my small purse into my carry on bad because they said it was a bag. I fly a lot, something like that had never happened to me before. But this was my first budget airline experience, so fair enough. It was no problem to put my purse in my carry on as I still had space, but it was a negative effect. It made me avoid the airline for many years

My second experience was a flight to London. I was visiting my friend short notice and their prices were unbeatable, so I decided to give Ryanair a second chance. I studied their rules meticulously and also noticed the very small personal item size. I only went for weekend, so a small backpack should have been enough. But even my small backpack was too big in one dimension. I was worried I could run into trouble and purchased their cabin baggage upgrade, which also includes priority boarding. It was still a very affordable flight.

To my surprise my experience was the complete opposite of my first experience. Noone cared about my bag at all! I was almost in disbelief. It wouldn't have been an issue to not buy the upgrade, but I really did not want to risk it. They did not check any bags of the people that had priority boarding. I can say about the people who didn't buy the cabin baggage. Everything was smooth and pleasant. I'll definitely use them again soon.

What I didn't fully get from your question is, did you book your flight with another airline, but it's operated by Ryanair (code share), or are you flying over to Europe and then will do some separate traveling with Ryanair? If it's code share the baggage rules of the airline you booked with apply. Otherwise of course only Ryanair's rules apply.

Concluding my comment I'm agreeing with what others stated. If your luggage is soft and you can squish it you will be fine. Ryanair has these metal frames in the two cabin baggage sizes at the gate. You must be able to fully fit your bag into the corresponding metal frame in order to be allowed on board with it, in case they check luggage size. As long as you can squish it in there you will be good.

If you are unsure if your luggage will be ok you can check for yourself at an empty gate and empty your luggage so far until it will fit. When boarding you need to carry the items that you had to take out on your body. It's not uncommon that people traveling on budget airlines wear a few layers of clothes on top of one another to save luggage volume. I think chances are good that you will be ok, OP.