The significance of the title of this poem lies in the way that it captures and inspires the imagination, allowing the reader to summon up an image of pastoral beauty which stands in direct contrast to "the roadway" and "the pavements grey" referred to in the last stanza. There is a sense too in which the word "Innisfree" suggests freedom and a lack of restrictions, which clearly fits into the theme of the poem as a speaker imagines a location where he can be restored by the power of nature against the corrupting influence of modern materialistic society.
In terms of developing this contrast there is too a significance with the fact that this imagined place of natural beauty is an island in the middle of the lake. It is, therefore, incredibly separate from the world that the speaker is fleeing from, and the fact that it is separated by water seems to emphasise that separation further. The importance of the water is focused on in the final stanza, where the sound of the "lake water lapping" is what the speaker thinks about to keep his heart pure in the middle of the materialistic city where he lives and works:
I will arise and go now, for always night and day
I hear lake water lapping with low sounds by the shore;
While I stand on the roadway, or on the pavements grey,
I hear it in the deep heart's core.
Note the alliteration in "lake water lapping with low sounds," which also has something of an onomatopoeic effect in creating the sound of the lake water as it laps against the shore. The title is therefore important in creating an image of this place of natural beauty and the antidote that it represents to the city life and materialistic society that the speaker finds so oppressive.