Yeats had a lifelong interest in mysticism, spirituality and astrology and much of his poetry was influenced by this, particularly Irish mythology.
In March 1890 Yeats was initiated into the occult group the Golden Dawn. After it broke up he remained with its offshoot the Stella Matutina.
Here is one of my favourite poems by Yeats:
Down by the Salley GardensLinks:
Down by the salley gardens my love and I did meet;
She passed the salley gardens with little snow-white feet.
She bid me take love easy, as the leaves grow on the tree;
But I, being young and foolish, with her did not agree.
In a field by the river my love and I did stand,
And on my leaning shoulder she laid her snow-white hand.
She bid me take life easy, as the grass grows on the weirs;
But I was young and foolish, and now am full of tears.