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.