James McFarlane, Neo-modernist Drama: Yeats and Pirandello, in Modernism: A guide to European Literature 1890-1930 , ed. Malcolm Bradbury & James MacFarlane (Harmondsworth: Penguin 1976, 1991) [Part 2].
In the view of Francis Fergusson, the most interesting writers for the stage between 1918 and 1939 - among whom he includes Yeats, Eliot, Cocteau, Obey and Lorca - find in these new sources the opportunity for a completely fresh start: The influences of the Moscow Art Theatre, the Ballet, and the Music Hall, combine to produce a new conception of the theatrical medium. Not only nineteenth-century naturalism, but most European drama back through the seventeenth century, is explicitly rejected in favour of medieval farce, Greek tragedy, peasant rituals and entertainments (Jamess Idea of Dramatic Form, in Kenyon Review, Vol. V, 1943, 506-07). Yeatss deep-rooted impatience with the constraints of naturalism found in these and related things a new source of strength. Taking his cue from Ezra Pound, and finding seminal inspiration in the Japanese Noh plays, he invented (as he claimed) a new form of drama in his Plays for Dancers (though he was quick to acknowledge that this newness was in a sense age-old): My blunder, he wrote in 1916, has been that I did not discover in my youth that my theatre must be the ancient theatre that can be  made by unrolling a carpet or marking out a place with a stick, or setting a screen against the wall (Note to Four Plays for Dancers).
Over the first twenty years or so of his dramatic authorship - from The Countess Cathleen of 1892 to the publication in 1911 of his Plays for an Irish Theatre - there was little of distinction (or even, apart from the dedicated Irishness of the works, of distinctiveness) to mark Yeats out among his European Symbolist contemporaries. Implacably anti-naturalistic, and unremittingly hostile to the angular problematics of Ibsen and Shaw, he hated A Dolls House: What was it but Carolus Duran, Bastien-Lepage, Huxley and Tyndall all over again? I resented being invited to admire dialogue so close to modern educated speech that music and style were impossible [...] As time passed Ibsen became in my eyes the chosen author of very clever young journalists who, condemned to their treadmill of abstraction, hated music and style. [...] He stood aghast at the sheer energy of Arms and the Man - which seemed to him to offer only inorganic, logical straightness and not the crooked road of life. (Autobiographies, pp.179 & 283.) He denied that drama had any business with reason or proof or the onward march of an argument; and asked that it should come flooding over us like some great tide, the drowner of dykes, the confounder of understanding. It must turn its back on all things that can be codified for ready understanding; its way was to move us by setting us to reverie by allowing us almost to the intensity of trance; the spectator must feel his mind expand convulsively, or spread out slowly like some moon-brightened image-crowded sea. (The Tragic Theatres, in Essays and Introductions , 1961, p.245.) To set drama to work pleading the National Cause, or insisting on the Ten Commandments, or discussing abstractions or the immediate circumstances of life or the things of the brain only was an abuse; the theatre should be the home of mysterious art doing its work by suggesting, not by direct statement, but by complexity of rhythm, colour and gesture.
For him, as for Maeterlinck and Strindberg, the fairy tale exerted a deep fascination; from it he fashioned a mode of drama both delicate and powerful. In The Countess Cathleen he turned, not for the last time, to Irish medieval legend; and although the atmosphere of the setting in cottage and castle - the portentous calls of animals and birds, the obtrusive omens - give the impression of being derivatively Maeterlinckean, it is doubtful whether at this early stage in his authorship Yeats was in any way familiar with Maeterlincks work,  though that familiarity grew swiftly, both from the theatre and from the printed page, as the decade progressed. A sense of quiet brooding and of intense longing, the simple profundities and the eloquence of silence create for these early dramatic works a world of Maeterlinckean outline; it is peopled by characters greatly preoccupied with their search for the one scene, the one adventure, the one picture which is the image of their secret lives. Intensity, simplicity, quietude are their goals, yearning as they do for that far household where the undying gods await all those whose souls have become simple as flame, whose bodies have become quiet as an agate lamp ('The Philosophy of Shelleys Poetry, in Essays and Introductions , p.95.)
His hostility to the use in drama of the pallid phrases of modern educated speech was matched by a faith in the poetic resources of Irish peasant speech; his own (and, following his encouragement - Synges) use of the rhythms and images of spoken peasant language was the first of a number of attempts over the following years elsewhere in Europe to do something comparable - the supreme example being of course Lorca, but including also some of the earlier works of Pirandello. Yeats came very soon to recognise also that the achievement was not merely the replenishment of the sources of dramatic language but the articulation of values and views which would otherwise have remained unexpressed, the opening up of new and previously unplumbed deeps of the mind. When in 1919 he reflected on what he and his fellows had achieved by their endeavours, he defined it as the first doing of something for which the world is ripe, something that will be done all over the world and done more and more perfectly, the making articulate of all the dumb classes each with its own knowledge of the world [...] (A Peoples Theatre, in Plays and Controversies, 1923, p.206.)
To give assent to Edmund Wilsons much-quoted paradox that Yeatss greatest contribution to the theatre was not his own plays but those of Synge (whom, in 1896, he discovered stagnating in Paris and induced to return to Ireland) is to focus attention too exclusively on the plays of his Abbey theatre period and to neglect the very great seminal significance of the later dramas from 1916 onwards. For, as Eliot rightly remarked, Yeats only found his right and final dramatic form in the Four Plays for Dancers and other plays of his later years. Two things combined to give them their distinctive character. On the one hand there was the fact - which Yeats would surely be the first to give assent to - that his was a mind of promiscuous  habits: I have always, sought, he admitted, with considerable understatement, to bring my mind close to the mind of Indian and Japanese poets, old women in Connaught [sic], mediums in Soho. A roll-call of those who at one time or another had held his attention would have to include - alongside those with more obviously literary connections such as Blake and Shelley, Maeterlinck and Villiers de IIsle Adam and the French Symbolists - the names of thinkers and mystics and works from Plotinus and Boehme and Swedenborg and the Kabbalah, to Mme Blavatsky and the Theophists, Rabindranath Tagore and the Japanese Noh plays. From the complex chemistry of these elements were produced Yeatss mature views on the proper role of drama. As one critic has put it:The forms of Irish myth, the ideas and cosmology of Buddhism, and the dramaturgy of Noh would restore drama to its original sources and theatre to its only valid function, the evocation of a sacred presence. (Thomas Parkinson, The Later Plays of W. B. Yeats, in Modern Drama: Essays in Criticism, ed. T. Bogard & W. I. Oliver, NY 1961, pp.388-89.)
The other realisation was more formal. The mature Yeats, like the mature Strindberg before him, came to recognise that the sheer bigness of the contemporary theatre inhibited genuine drama. He found that the theatre with its mass audience was not giving him a sympathetic hearing; he felt himselfsitting behind the wrong people; he realised he was beginning to shrink away from the forbidding elaborateness of organisation, the man-management side, the daily rehearsals. Like Strindberg, he then gave himself to the creation of intimate theatre, to the composition of chamber plays, with few props and no stage machinery, communicating not to a mass audience but to a small and discriminating group. Heinvented a form of drama, distinguished, indirect, and symbolic and having no need of mob or Press to pay its way - to use his own words. (Certain Noble Plays of Japan, in Essays and Introductions, 1961, p.221.)
The over-riding concern was to achieve a distanced intimacy: to counterbalance this new intimacy of setting with a new and organic separating strangeness. This strangeness was entirely different from thebodily distance which the mechanicalness and noise of the contemporary theatre created; it was to be achieved by,human means, by ritual, stylisation, the formalisation of the dance, by abstractive transposition into music, by the depersonalisations of the mask - the creation not of artificial worlds of elaborate sets and of reproduction reality, but of a distanced world with its own inherent authenticity (Ibid., 224-25.) 
All imaginative art remains at a distance and this distance once chosen must be firmly held against a pushing world. Verse, ritual, music and dance [...] must help in keeping the door [...] The arts which interest me, while seeming to separate from the world and us a group of figures, images, symbols, enable us to pass for a few moments into a deep of the mind that had hitherto been too subtle for our habitation.
The innovatory significance of these later highly formalised dramas. sustained as they were byverse, ritual, music and dance, the imaginative use of masks and the consequent diminution in the importance ofcharacter - a complex term in Yeats - did not win immediate critical recognition. But they can now be seen to have played a decisive part in creating conditions wherepoetic drama - a term quick to take on adistinctly effete and pejorative meaning (J. Chiari, Landmarks of Contemporary Drama , 1965, p.81) - was once again possible in Europe; and to have inaugurated the intensive exploration of a number of elements in drama that had long suffered neglect. The later theatrical tradition of Waiting for Godot and Fin de Partie [ Endgame ] - it is argued - can be seen to have its origins in At the Hawks Well , The Cat and the Moon and The Hernes Egg. (End; pp.562-70.)