It may not sound plausible, nor even safe, but nevertheless, you can get a bus to and from Kefalonia either from Patras or from Athens. The prices are extremely reasonable, and, obviously enough, include the price of the ferry crossing.

It is quite a long journey, however. A bus leaving Athens at 5pm, say, has been known to turn up at the station in Argostoli after 2am the next day!

KTEL Kefalonia:
The bus station is about 50 yards from the bridge in Argostoli, on the road out of town round the lagoon.