. I'm sure when your bank has to change the currency themselves, they charge you for it
They do indeed, but it's likely to be cheaper than Ryanair's currency exchange rate.
Have a read of this which should save you a little bit.
Also if you scroll down to the section " Is there any other way to avoid currency conversion " it explains what you asked in your thread question, but unfortunately it's not possible to do this anymore since Ryanair updated their website.