Getting to Iona is somewhat of a journey. From Oban you can take a car ferry to the island of Mull (one small car cost approx. $90. for a roundtrip). The trip takes about 1 hour and arrives in Craignure. From Craignure take the one-track* road to Fionnphort (approx. 2 hours). At Fionnphort you can leave your car and take the passenger ferry to Iona (approx. a 20 minute ferry ride). The ferry from Fionnphort to Iona runs about every 30 minutes. Weather conditions may affect this schedule.
*Note: While traveling on the one-track road be sure to pull over to the allocated spots on the side of the road when another car is approaching. Beware, the tour buses are ruthless!