Heads up in case you're booking with Ryanair using a currency that's not your usual (i.e. if you book a one-way flight from somewhere to come back home again):

Their guaranteed conversion rate is somewhat off the actual conversion rate. Taking it will increase the price you pay by a noticeable number of quid, as you can see in the two bookings I just made (the Ryanair conversion rate was charging £40.49 instead of the actual conversion amount of £37.96, and £27.03 vs the actual conversion amount of £25.06)

The comparison of Ryanair's 'Guaranteed Exchange Rate' vs what the actual costs were

Now it isn't TONNES off, and if you're likely to get stung on conversion fees elsewhere then it might be worth biting the bullet and taking the guaranteed conversion from Ryanair, even if it is a few quid more.

But, if like me your bank / however you choose to pay doesn't charge mad conversion fees, it's probably worth avoiding Ryanair's inflated guaranteed conversion rate (e.g. I paid £38.06 instead of the actual price of £37.96, whereas Ryanair wanted to charge me £40.49. So by letting my bank do it (which I know has very good conversion rates and no fees for it), I saved £2.43. Doesn't sound like much, but it soon adds up (and technically is a saving of around 6%)

Just something to bear in mind!