I am looking for the use of myth in "Easter 1916."

"Easter 1916" is a poem written to and for the Irish people at the time of the Easter Rising. As such, it contains many references which would have immediately made sense to the intended audience but are now layered and cryptic. One of these is the reference to the "winged horse," an allusion to Pegasus in classical mythology. In the nineteenth century, Pegasus became associated with poets and poetry; it is for this reason that Yeats makes the connection between the "winged horse" and the man in the poem who "had kept a school." This is a reference to the poet and schoolteacher Padraic Pearse, writer of other great poems of the Easter Rising such as "The Wayfarer." Pearse was a significant figure in the uprising.
There are no other obvious allusions to classical mythology in this poem, but in terms of symbolism, we can point to Yeats's reference to "wherever green is worn" as alluding to Irish legend and folklore, within which context green is strongly representative of Ireland and the Irish people.