What is the relationship of the Celtic gods and the sidhe (aos si)/annwn?

Prince Arthur and the Fairy Queen.
 (Photo credit: Wikipedia)

From reading the mythological cycle of Ireland the Deities were forced out by the Sons of Mil and from folklore they enter the sidhe. The sidhe/annwn over time becomes the home of the fairies and others like the fairies with less connection made to the gods. I am interested in how others view the this connection between the sidhe is the location of the Celtic deities.