OR LONG periods of his life William Butler Yeats lived with the unsettling suspicion that he was reliving old myths, that his life and all lives are caught up in great cycles of history. Even if he could understand those cycles, he felt both guided and doomed by them. That theme runs strongly through much of his poetry, particularly in the three blank verse plays which open at the Ex' tonight. Written over a period of thirty-five years, they each present a different way of looking at a hero, in three stages of his life. The particular hero is the warrior Cuchulain, whose life and character Yeats drew from Irish legend and modified many times in his poems and plays.

The bare outline of the legends: young Cuchulain, challenged by the warrior-Queen Aoife, defeats her in battle and that night begets a son by her. Years later, after taking an oath of allegiance to his king Conchubar, Cuchulain's son appears as a nameless young man who challenges Cuchulain at Conchubar's insistence, and is killed by his father. Learning the young man's identity, Cuchulain turns on Conchubar but is charmed into attacking the waves at the seashore instead. Finally, in his old age, weakened from loss of blood, Cuchulain meets Aoife again--who has come to kill him, and does.

Yeats does much more than retell these legends; he loads them with the iconography of his mystical theory of the cycles of history and the phases of the moon and with symbolism too rich for simple explanation. Because of their roots in Irish legend, Yeats's plays have never proved popular to audiences beyond the Liffy.

At the Hawk's Well begins just before young Cuchulain's first meeting with Aoife. The play is done partly in the style of the Japanese Noh drama, a form that intrigued Yeats for its simplicity and ritual: it begins with the ceremonous unfolding and folding of a large cloth, a less mechanical and more suggestive device than a curtain for marking the division between the presence of actors and the progress of a play. William Barnum's slow and deliberate opening mime as the Old Man at the elusive fountain of immortality does more to set the scene than the other actors are later able to maintain. Barnum and Wally Know as the Young Man (Cuchulain), use thir mature voices to form a firm center of dialogue in a production that's otherwise weak in intonation and uncertain in supporting roles.

BARNUM AND KNOX complement each others' strengths again as Conchubar and Cuchulain in On Baile's Strand, but this play uses the relationship of two lesser characters, the Blind Man and the Fool, to equal purpose in commenting on the progress of Cuchulain's life. Peter Wirth and Joel Davidson succeed only partially in filling these two roles with intelligent but unrealized interpretations. Director Donnally Miller emphasizes the mutual dependence of the two half-men well enough, but the scenes where they're alone, ideal for comic improvisation, drag more than they should.

With the prologue to The Death of Cuchulain, Yeats speaks directly to his audience through the character of the Old Man--a figure Yeats created shortly before his own death. Miller has taken the cue to dress Barnum, (who already fairly resembles the old poet) in a suit and spectacles Yeats might have worn himself. The impersonation of the hoary and slightly confused Yeats is the strongest performance of the evening. It makes it clear that the poet wrote a part of himself into each of the Old Men in the other plays.

In putting these plays on stage, one of the most important challenges to be met is providing a sense of internal unity and discipline among the actors, dancer, and musicians who combine in abstract and occasionally mystical scenes. These demands on the performers were of primary importance to Yeats, who intended to impress his audiences with the significance of mythical symbols through purely visual experience. That sense of unity was not nearly strong enough when the production opened in Leverett House last weekend. With a cast better-practiced by a week, the real problem raised by this trilogy may seem not whether Yeats can be performed well, but something much stickier: whether anyone who doesn't love Yeats can really like his plays.