This poem is about the dichotomy of the thinker and the actor. Yeats, in love with Maud Gonne, was the thinker, the courtly lover -- the one who would "brood upon love's bitter mystery." Yeats was Mr. Nice Guy. Yet Yeats wanted to be the actor - the alpha male - the Fergus. Note the sexualized subtext that permeates the poem, who will "pierce the deep wood's woven shade"? Who will "drive" with Fergus. Finally, we get the reasons to be the alpha male - the man of action, in the repetition of the word "rules." The alpha commands and takes what he wants.
• I'm not sure if Fergus is man or God as the last four lines talks of his rule over woods,sea and stars.
Well for me Yeats is asking his readers to model Fergus's actions. He renounced all materialistic desires (including love) and sought a life of simplicity and spirituality, and danced upon the level shore because of it. The deep wood's woven shade = the unknown. And in response to the previous comment, in my opinion I think that "brazen cars" is in reference to battle/warfare.
Summary
The poet asks who will follow King Fergus' example and leave the cares of the world to know the wisdom of nature. He exhorts young men and women alike to leave off brooding over "love's bitter mystery" and to turn instead to the mysterious order of nature, over which Fergus rules.
Analysis
This short poem is full of mystery and complexity. It was James Joyce's favorite poem, and figures in his famous novel Ulysses, where Stephen Daedalus sings it to his dying mother.
On one level, the poem represents Yeats' exhortation to the young men and women of his day to give over their political and emotional struggles in exchange for a struggle with the lasting mysteries of nature. He suggests that Fergus was both brave and wise to give up his political ambition in exchange for the wisdom of the Druids, as depicted in the poem "Fergus and the Druid." Of course, from that poem we know that Fergus' sacrifice was