On the Ryanair website, where you key in your credit card details, it will show you the price, in this example GBP as the flight is originating in the UK. When you key in your credit card number, Ryanair can tell if it is a Euro credit card. It so, then it shows you their euro equivalent. Next to this in red writing it says something like (More about this). Click on this and a box will pop up telling you the rate they are using for conversion and it recommends that you keep this 'guaranteed rate'. At the end of this is a tick-box which you can untick. You will then pay the Euro equivalent based on your Credit Card issuers rates rather than Ryanair's extortionate exchange rate.
I have done this a number of times and the difference is quite substantial.