Is this the ad about 'The Dream Club'? It's an exercise in surrealism, and the subtle link between a man who can dream the dreams that the other 'members' envy, and the Guinness is either his inspiration, or his reward - it seems to be open to interperetation. Life-size squirrels are the sort of bizzare characters that crop up in dreams, and add to the whole ethereal atmosphere. Guinness has a history for making ground-breaking memorable adverts. This is just one more in a long line.
The squirrel appears when the person wakes up and finds he is a squirrel. So is a squirrel dreaming he was a person or a person dreaming he was a squirrel?