didn't show up until after the final payment was made
That would be theft -- if you authorise payment for one amount, they can't add charges to it afterwards. I think what you mean is that, after entering all your details, just before you confirm, they mention the credit card charge. You still have a chance to cancel the transaction before proceeding (I think -- unless they have changed the procedures recently!).
They do mention the charges in their terms and conditions. The charge depends on how you are paying -- e.g. fees for debit cards are lower. So, until you enter your payment details, they can't display the correct charge for you.
As to whether the charge is excessive -- probably. I would be surprised if credit card transactions cost them more than 1% of the transaction value to process (but remember the transaction includes taxes and airport charges, which don't contribute to their bottom line.). So some element of their profit must be coming from this charge -- it's a bit like ebay sellers who make their money on "postage". But the total cost is still good value, so we don't complain, even though the sleight-of-hand makes us feel ripped-off.
Another avenue of complaint is that the card organisations (Visa, MC, Amex) normally expressly prohibit credit-card surcharges. Ryanair are probably in violation of their "merchant agreement" with their "acquiring bank(s)". It may be possible to reclaim the fee from your bank on this basis.