The poem is a dialogue between the aged Irish hero Oisin (pronounced Usheen) and St. Patrick, the man traditionally responsible for converting Ireland to Christianity. Most of the poem is spoken by Oisin, relating a supernatural journey he has taken through three enchanted islands. Having begun the journey as a young man, he returns to find his warrior companions dead, and the pagan faith of Ireland being displaced by Patrick's Christianity.
Oisin has not been a popular poem with critics influenced by modernism, who dislike its pre-Raphaelite character. However, Harold Bloom defended this poem in his book-length study of Yeats, and concludes that it deserves reconsideration.