One said that he thought you'd be able to as it is a big ferry and they take lorries too
The other I've copied and pasted for amusement:
"i'd be surprised if you can't I have only taken the bike over as a foot passenger but as jason said there's lorries and electricity and everything It's still 1952 on the island but the transport to get there is relatively modern"
I know with some of the ferry companies, you get asked at the booking stage. Definitely the case with Isle of Man (Steamboat Packet Ferries) but that's because they pack the cars in so tightly, they need to know which cars they can put under the lower roofs (especially if it's the catamaran). Can't remember with CalMac.