Ryanair does not operate Frankfurt-Main to Lisbon.
There is a Ryanair service from Hahn Airport (100 km from Frankurt) to Faro Airport in the Algarve, a couple of times a week. There are regular buses from Hahn to Frankfurt centre. Ryanair also has services from Hahn to Porto. From either of these centres you can get trains or buses to Lisbon.
TAP/Lufthansa have the route Lisbon - Frankfurt-Main tied up.
You might get a better deal to Lisbon on AirBerlin.com via their hub at Palma da Mallorca.
TuiFly.com/Hapaglloyd Express operate joint flights on the Frankfurt-Main to Faro route.
