When it comes to using Irish mythological figures, Yeats pulls out the big guns: Cuchulain is one of the most well loved figures in the Irish canon. And he was pretty kick butt. In one myth, he even rips a dragon's heart out through its throat. Yowza. (Source.)
Countess Cathleen wasn't just a character in Yeats' play of the same name. In fact, the woman was inspired by a character from his own love life, Maud Gonne. Gonne was a rebel of the first order. Feminist, writer, and Irish nationalist, she drew Yeats into Ireland's struggle for independence from the British. (Source.)
Twentieth-century authors love stealing lines from Yeats's poetry for the titles of their books. We can think of at least three books that took their titles from this poem alone. The American novelist Cormac McCarthy wrote a novel called No Country for Old Men (since turned into a Coen Brothers movie), and another famous American novelist, Philip Roth, titled one of his books The Dying Animal. Both titles were borrowed from the Yeats poem "Sailing to Byzantium". And Chinua Achebe's famous novel Things Fall Apart gets its title from what is perhaps Yeats's most famous poem, "The Second Coming". Plus, the famous American essayist Joan Didion named one of her essay collections Slouching Towards Bethlehem, which is also taken from "The Second Coming." Phew.