It happen to me also, I normally look very carefully at the Ryanair website and avoid all the charges with my prepaid credit card.

At the very end, when you put in your credit card number it determines which currency the credit card is in, and in very small blue text it shows the forex rate which is going to be charged, and you have to click at the link to cancel this and let your credit card company do it yourself.

For me it was not about the cost, it was a 5 pound ticket, so probably only cost me 20 eurocents or so, the bad thing about this that you actively have to click the proposed conversion away, and the black text shown the amount to pay does not change.
It should be the other way around, Ryanair is free to make money on this one as well, but should be optional to the customer and the customer should actively click to allow this.
(There are some cases, especially with US credit cards where their company charge an extra fee for paying into another amount then US Dollars, where this is actually helpfull and saving the customer money).