Has anyone done this in one day? It looks feasible and more interesting than taking the bus.
I don't believe there is direct ferry from Galway to Doolin. At least nothing came up when I googled it, unless you want to go via the Aran Islands (ie from Galway to Inishmore to Doolin). The Doolin2Aran ferries offer a BUS service between the two, but that stops at every little town along the way (as does Bus Eireann, I would assume). Driving a rental car would take less than 2 hours, so if you are interested in fastest, that would be the way to go.
I took a ferry to the Aran Islands. The water is so rough, I got sick on the boat and it ruined my whole day. I wouldn't take a ferry unless you had to.