Be aware that Ryanair does not use LHR or FRA.

They use STN and Hahn, around one hour further out than FRA, with a payable bus connection to/from Frankfurt main rail station. Taxis and car rental also available, I believe.

One problem with Ryanair, as noted several times in these boards, is that they are often optimistic (= misleading) with their destination names.

Not Frankfurt but Hahn.
Not Brussels but Charleroi.
Not Stockholm but two separate (and distant) alternatives.
Not Paris but Beauvais.
etc etc

They do use some 'real' airports, such as Turin, so sometimes not misleading!