The Irish poet William Butler Yeats was one of the giants of 20th century literature, and he frequently used myth, legend, and symbolism in his poetry. While he was closely tied to his Irish roots, he drew from a deep well of sources and, in these poems, there are more examples of world mythology and history than Irish.
In "Sailing to Byzantium," for example, the primary image/symbol is that of the ancient city of Byzantium, a Greek colony that would later become Constantinople, the seat of the Byzantine Empire. In the poem, which is about, among other things, spirit and flesh, the city is a symbol for something grand, immutable, and beautiful.
Yeats will use the city again in the poem just titled "Byzantium," although this poem, while still concerned with the spirit and the flesh, is less transcendent, with its more earth-bound images of drunken soldiers and forges. Hades, the Greek underworld, is mentioned twice, so, again, I would argue that Yeats's imagination is more global than Irish.
"Leda and the Swan" also draws from Greek mythology, in this case the story of Zeus taking the form of a swan and raping Leda, who then gave birth to several children, notably Helen of Troy, whose abduction launched the Trojan War.
Finally, in "Lapis Lazuli," Yeats uses the blue stone as his primary symbol, while also alluding to characters (Hamlet, Lear) from Shakespeare. Better examples of Yeats's Celtic side are "Fergus and the Druid," "Who Goes With Fergus?", and "The Song of Wandering Aengus."