Theory:

And I shall have some peace there, for peace comes dropping slow
Dropping from the veils of the morning to where the cricket sings;
There midnight’s all a glimmer, and noon a purple glow,
And evenings full of the linnet’s wings.
Explanation:
 
The second stanza explains why the speaker desires to take up the journey to Innisfree. He associates the isle of Innisfree with peace. He says that he "shall have some peace there". Furthermore, the speaker describes the manner in which peace arrives. He says, 'peace drops slowly' as the night breaks into dawn. The speaker uses imagery to explain the concept of peace here.
 
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Peace comes dropping slow from the veils of the morning

Cricket's song is usually associated with nighttime. Hence, the speaker is trying to paint the image of dawn where the sun rays can be seen slipping slowly through the skies to reach the earth that is still dark (as the cricket could be heard singing). Peace drops slowly from the veils of the morning to the earth that is still asleep.
 
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A cricket

Innisfree is nature bound, and hence, time seemingly moves slower there than in the city. Thus, the peace that the speaker years for is both slow and tranquil. Unlike the city, the night at Innisfree is filled with glimmer, probably from the moon, stars, or even fireflies.
 
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The midnight is all a glimmer
 
The noon brings a purple glow, and during the evenings, one would hear linnets fluttering around.
 
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Purple sky during he noon (left); Evenings filled with the fluttering linnets (right)
 
Meanings of difficult words:
 
S.No
Words
Meanings
1
FlutterTo flap the wings
2
VeilA curtain-like thing
3
CricketA type of insect known for its chirping notes produced during the night
4
Glimmerto shine faintly or unsteadily
5
LinnetA small bird known for its musical notes
6
TranquilFree from disturbance or turmoil; calmness
Reference:
National Council of Educational Research and Training (2006). Beehive. The Lake Isle of Innisfree- W. B. Yeats (pp 54). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.