In his preface to the play (published with three others in Four Plays for Dancers), Yeats has this to say about the set design:

The designs by Mr. Dulac represent the masks and costumes used in the first performance of "The Hawk's Well." The beautiful mask of Cuchulain may, I think, serve for Dervorgilla, and if I write plays and organize performances on any scale and with any system, I shall hope for a small number of typical masks, each capable of use in several plays. The face of the speaker should be as much a work of art as the lines that he speaks or the costume that he wears, that all may be as artificial as possible. Perhaps in the end one would write plays for certain masks. If some fine sculptor should create for my "Calvary," for instance, the masks of Judas, of Lazarus, and of Christ, would not this suggest other plays now, or many generations from now, and possess one cannot tell what philosophical virility? The mask, apart from its beauty, may suggest new situations at a moment when the old ones seem exhausted; "The Only Jealousy of Emer " was written to find what dramatic effect one could get out of a mask, changed while the player remains upon the stage to suggest a change of personality. At the end of this book there is some music by Mr. Rummell, which my friends tell me is both difficult and beautiful for "The Dreaming of the Bones." It will require, I am told, either a number of flutes of which the flute-player will pick now one, now another, or an elaborate modern flute which would not look in keeping. I prefer the first suggestion. I notice that Mr. Rummell has written no music for the dance, and I have some vague memory that when we talked it over in Paris lie felt that he could not without the dancer's help. There is also music for "The Hawk's Well" by Mr. Dulac, which is itself an exposition of method, for it was written after a number of rehearsals and for instruments that have great pictorial effect.