"The Green Helmet" by William Butler Yeats is not a play, but a collection of poetry. It was written after his collection "In the Seven Woods", and is a continuation of that style. The poems are, in general, about how difficult it is to be a poet, and how difficult it is to be a human being. Yeats writes several poems about Maud Gonne, a women he was in love with, but calls this a "barren passion" and is bitter and spiteful because he was rejected. The collection is a transition from the poet's earlier lust for beauty and wonder, to his spiteful disappointment in the world.
The prior answer is incorrect. The Green Helmet --is-- a play, and was put on in the Abbey Theater, if memory serves. It was published as PART of a tome which included poetry, but it is one of Yeats' several works dealing with the Cuchulain myth.
It is a short play, and relatively easy to read, despite being in verse. The short summary is: Cuchulain comes back from his wandering and battling. He finds out that one of his pals owes a Red Man (a supernatural force) a head -- the Red Man let his be lopped off. It turned out that the Red Man didn't need his head (parallel to Gawain and the Green Knight). After arguments and calamities, all of which Cuchulain calms down, Cuchulain offers to pay his friends' debt. the red Man rewards Cuchulain with the championship of all Ireland.
The play is found on page 51 of the text as legally svcanned by Google (it is out of copyright). Read it. It is well worth the effort, especially the Red Man's final speech.