The official CalMac advice is to travel earlier to maintain connections. West Coast Motors can't hold the afternoon bus to Fionnphort because it connects with the last ferry to Iona, due to crew working hours that ferry can only be held for 10 minutes for a late running bus.
Thus to Iona I would not travel on the 1555 sailing from Oban, but on the previous Loch Frisa sailing. I would therefore leave Glasgow on the 1036 train, arrives 1346, then the 1415 ferry over to Craignure.
It's a good job that most of the school kids from that end of Mull board in Oban during the week, so are not trying to commute home on the 1555 ferry. That will be more of a problem for the Tobermory children who do commute.
On the way back the local Ferry Committee do not expect that you will be able to catch the 1211 train from Oban, you need to book on the 1441 train. If you make the 1211 train then it will be a real bonus.
We now have a further added note from Calmac that a luggage van will travel on these sailings- Depart Oban - 09:55, 12:15 and 15:55, Depart Craignure - 11:05, 13:35 and 17:05. So you are expected to put your luggage in the van and collect it at the other end- to further delay things.
With only 2 berths at Oban if the big ferry to Mull is consistently late that could also have knock on affects for the Loch Frisa sailings, and departures to all other islands (Colonsay, Tiree/Coll, Barra and Lochboisdale).