I've never really given early Yeats much attention, picking up on him around the time of "The Wild Swans At Coole" with only a very few exceptions – "The Lake Isle of Innisfree" – for example. …
So when my friend Mary selected a number of early poems to serve as an provide an aesthetic underground for her continuing development of Musil's "The Man Without Qualities", I came to them with very little prior knowledge.
Which makes understanding "Cuchulain's Fight with the Sea" quite tricky.
It doesn't help that I had no knowledge whatsoever of the Irish mythology surrounding Cuchulain, having never been taught about such matters in a British high school. (Plenty of Greek and Roman mythology though!) Yeats was very much part of the attempt of Irish intellectuals in the 19th and 20th centuries to rescue the legends and myths of that island from obscurity, all contributing to a crystallizing sense of Irish identity that would eventually lead to political independence.
So here we have a poetic reexamination of part of the legend of Cuchulain. There is one central point. This does not leap out at you, but is made clear by allusion and inference. Cuchulain kills his son, unknowing of his identity until his death. This brings upon him a kind of murderous madness that is deflected by the magical devices of the Druids into a futile battle with the sea.
The death is a consequence of masculine pride in both father and son. But it can be considered further as an internal battle between two aspects of the same personality – hence the shared name and the shared oath – and a stark warning that the overmastering of one by the other leads to misery and impotence. Only as balanced and complete individuals can we thrive. Destroy that balance and you cripple yourself.
Many of Yeats' poems deal with a transformative incident that forever changes the fate of the subject. This is one of the starker and unsettling ones.