A few years ago, the Irish airline Ryanair put a former U.S. air base to use as an established base in the heart of Europe. Now, it's one of the busiest airports on the continent. The downside is its distance from Frankfurt, 120 km, or 90 minutes by bus. I recommend the Bohr company bus at the terminal exit. We booked online tickets at Terravisión, another bus company, and waited half an hour to leave, and the informality of the driver cost us another half hour at night in bad, cold weather, since the airport doesn't have a bus station shelter. So take warning.
The Irish airlines Ryanair began to reuse a former U.S.