There is no general rule here - you need to look at specific airport codes you want to fly to. Don't trust airport names used by Ryanair (their "Frankfurt"-Hahn airport is closer to Luxembourg than to Frankfurt, for example), but their 3-letter airport codes are correct.

And be aware of their restrictions - e. g. their free hand luggage allowance is smaller than on most carriers, and if you travel as a couple, you will either have to pay for seat reservations, or be likely seated apart, because you cannot select seats for free, even during check-in. Have a look at their "fees" page - it gives you an idea of the stuff you would have to pay extra for.