Ryanair baggage weight allowance is simple - 10kg carryon, 15kg checked in total.

Anything above that you pay £5.50 (€8) per kilogram

You also pay a fee per checked bag (£3.50 or £7) which is separate from the weight allowance.

Example if you have 10kg carryon & 15kg checked but your carryon is too big you'll pay £7 to check the bag plus £55 for the excess baggage as the fact that you are unable to use your carryon allowance is irrelevant