Ryanair is an Irish low-cost carrier that typically uses remote airports, as part of its business model, to keep down costs. When I went to Stockholm last year, Ryanair flew in to Skavsta Airport, near Nyköping, about 100 km (62.5 miles) south of the city. When we went to Milan, our plane flew to Bergamo which is about an hour away. 
To get to Frankfurt am Main, Ryanair makes use of the Frankfurt-Hahn Airport. Despite its name, the airport is over 120 km (~75 miles) west of Frankfurt.
What's funny is that the last time I was here, I was in the U.S. Air Force and it was called Hahn Air Base. During the Cold War, Hahn was home to the 50th Fighter Wing as part of the U.S. Air Forces in Europe (USAFE).
In 1993, the base was turned over to the German government and they turned it in to a regional airport. Now the flightline has Boeing 737s instead of F-16 fighters.
And who would have thought all of those years ago that a former NATO base would now be used for commercial flights to Prague?At least for now...Ryanair announced last week that they are going to pull out of Prague this Fall.