Why does the speaker vow to go to Innisfree, and how does the life he imagines living there compare to the one he is now living?
Yeat's "The Lake Isle of Innisfree" is an example of the nostaglic and pastoral impulse of the Celtic Twilight, a poetic movement which contrasted a notion of a spiritualized Celtic tradition, grounded in the myth of the past of the Celtic frings (Scotland, Ireland, Wales) with the modern, rational urbanism of England.
In this poem, as in "The Fascination Of What's Difficult", Yeats is speaking out of frustration with life and work in Dublin ("the pavements grey"), the urban environment most associated in southern Ireland with English rule, as it was the centre from which England governed Ireland in th 19th century.
In the poem, the narrator explicitly states that he is going because he will "have some peace there", for in that idyllic rural (and authentically Irish) environment lies the wellspring of his poetry "in the deep heart's core." Thus the vow and his fantasy about rural life are both connected with a simultaneous desire to be away from the city and return to the simple country life of a farmer in touch with the rhythms of the earth.