Yeah, ferry from Oban to Craigmure on Mull is the simplest way, only takes about 45 mins to do the crossing. If you want to do Iona, it’s about an hour on single track (but pretty well maintained) roads across the island and then a 10 minute ferry on the other side.
We actually went the long way round and drove up through Ardnamurchan to get the ferry at Lochaline because the Oban ferry was fully booked: meant we had the added bonus of seeing some absolutely beautiful scenery around there too.