I've said before that Yeats is one of my favourite poets. This poem will hopefully show you why.
Let's begin by listing the obvious themes that this poem is about: isolation, independence, peacefulness, solitude, contentment, carefreeness - there may be more. These are fairly prosaic themes, so why is it a great poem?
How construction contributes to the pace
First let's consider what Yeats does with the meter of this poem:
I will/ arise/ and go now,and go/ to In/nisfree,
And a small/ cabin/ build there,/ of clay/ and wat/tles made;
Nine/ bean rows/ will I have there,/ a hive/ for the hon/ey bee,
And live/ alone/ in the bee/-loud glade
I'm not always sure that I have scanned it correctly but I think I hear the effect correctly. That long foot in the middle of line one, coupled with the comma, drops a pause into the centre of that line. Then as you enter line two the iambs of line 1 give way, initially to trochees at the start of the line which serve as a brake as you read the line. Then the iambs come back to close the line. The fourth line is short by two feet. The whole effect is to create pauses as one reads. It deliberately slows the reader down, just like the pace of life on Innisfree Isle.
But it is not meter alone which slows the line in this piece. Also note the end-stopped rhymes of free and bee / made and glade which prevent a tendency to run the line on when reading and which close off the lines and stanzas.
Each line is heavy with alliteration or with internal rhyme as in S2's dropping, morning, sing, linnet. The effect that the alliteration and internal rhyme achieves is a different one, however. I'm sure that you will notice as you read it that alliteration has no appreciable slowing effect.
But what is it that Yeats wants to do on this Isle? He wants to plant beans, have bee hives, hear the waves lap the shore and watch the day pass. These are repetitive activities which are primarily the same from day to day with only minor variation. That's what alliteration is too and the ear hears the effect of that repetition as one reads.
A change of scene
You might also have noticed that the first two quatrains are completely devoted to describing the Isle. Then suddenly, without development or warning in the second last line of the third and last quatrain he introduces an entirely different place altogether: a roadway and a grey pavement.
This line and the last one demonstrate with enviable brevity that Yeats is not at his Lake Isle, he is somewhere which is very different indeed and the Lake Isle is a place he wishes for.
Now with that thought in mind, look back to L1. The poet says "I will arise and go now and go to Innisfree." And when he gets there - he will do all the thing set out in the poem. Now look back at the last quatrain: Always night and day he hears the water lapping. While he stands on the roadway he hears it. This clues us in to the fact that while Innisfree may be (and apparently is) a literal place - Innisfree is more than just a place to the poet. It is a symbol of an emotional way of being.
Emotions are typically abstract things but Yeats concretises his feelings for the reader by describing them as a place -and not just as any place, but a place with concrete specific details. We know what the cabin is made of, what he will plant, how many rows. Nor does he limit the description to narrative - he ties in other senses by describing what the glade will sound like (bee-loud, cricket's singing and water lapping) so that at the end he doesn't have to say he feels it or sees it but rather he hears it, in his heart's core almost like a heartbeat.