There are two alternative routes if you are driving to Iona. The most popular route is via Oban however there is also a route via Corran and Lochaline. The Oban route is more direct however the ferry crossing is longer and more expensive. The Lochaline route involves more driving but shorter and cheaper ferry crossings. If you are going via Oban and intend to take your car across to Mull you will need to book the Oban ferry and you will need to be at the ferry at least half an hour before the sailing. See www.calmac.co.uk for timetables, conditions and booking details.
From Glasgow to Oban:- Head west out of Glasgow on the M8 following signs for the Erskine Bridge and Crianlarich. Cross the Erskine Bridge and follow signs for the A82 and Crianlarich. At Tarbet keep on the A82 and keep following signs for Crianlarich. At Crianlarich turn left still following the A82 and now heading for Tyndrum. At Tyndrum turn left onto the A85 and follow signs for Oban.
Alternative route: At Tarbet take the A83 for Inveraray. At Inveraray turn right onto the A819 for Dalmally and Oban.
Both routes take roughly 2.5 - 3.5 hours. Lunch breaks recommended at The Real Food Cafe in Tyndrum or if you take the alternative route it's the Loch Fyne Oyster Bar just before Inveraray.
From Glasgow to Lochaline:- Same route as above except do not turn left at Tyndrum. Instead you should stay on the A82 and head across Rannoch Moor and through Glencoe. Cross the Ballachulish Bridge and look for signs for the Corran Ferry. Take the Corran Ferry (not bookable) and at the other side turn left and follow signs for Lochaline. Take the Lochaline ferry (again, not bookable) and at Fishnish on Mull you should head through Craignure and on to Fionnphort.