Upvote:3
Ryanair have two sizes for hand luggage:
Your backpack seems to be 46 x 33 x 15 cm, so it clearly exceeds the free “personal item” allowance (but fits in the larger one).
I have no recent experience with Ryanair, but other low-cost airlines like Easyjet which have a very similar policy have frequent checks for number or size of hand luggage. It seems to vary a lot between airports, but I would personally not take the chance: the additional bag allowance is usually much more expensive at the gate than it is when bought in advance.
And of course you don’t want to end up with the worst case scenario (there’s a limit in how many additional hand luggage allowances they sell on a flight), having to check your backpack at the gate, at last-minute checked luggage prices (probably even more expensive), and with the additional risk inherent to checked luggage.