Getting to and from Kefalonia
There are numerous ferries every day from the town of Kylini in the western Peloponessos about a 45 minute drive from Patras. You can find ferry schedules at www.ferryhopper.com
Kyllini is a not very attractive fishing port, agricultural village with a handful of restaurants and fast food joints, a few hotels, and a town beach and if you miss the last ferry there are worse places to be stuck. Unless of course you can't find a room and find yourself battling mosquitos all night waiting for the morning ferry. For this reason I suggest giving yourself
more than the 4 hours it takes to drive from Athens and aim for an earlier boat.
The airport at Argostoli has daily flights from Athens as well as charters from several European cities. For flights from Athens check the website of Aegean Airways. Most of the charters are for package tours so you may not be able to just get a seat on a plane unless they are having trouble filling it. There are flights to Kefalonia from London, Newcastle, Manchester, Birmingham, Milan, Pisa, Rome, Prague, Vienna and Innsbruck among other places. Easyjet flies there as well.