Why is Riders to the Sea by J. M. Synge considered a poetic drama?

J.M. Synge is famous among Irish authors for attempting to capture the unique sound and rhythm of the native Irish accent. To do so, he travelled on several occasions to the Aran Islands, a remote region celebrated for its traditional culture and customs. Synge supposedly based the dialogue in his plays on the Irish dialect he heard during his travels, and his efforts have drawn both praise and criticism. However, whether or not you believe Synge's writing to be an accurate representation of English spoken with an Irish accent, the language in his plays is undeniably poetic and lyrical, and so it would be accurate to call Riders to the Sea a poetic drama.

Take, for instance, this quote: "I'll have no call now to be up and crying and praying when the wind breaks from the south, and you can hear the surf is in the east, and the surf is in the west, making a great stir with the two noises, and they hitting one on the other" (42). This lyrical description, which takes place late in the play, is a small example of Synge's prodigious literary powers, and it describes the violence and ferocity of the sea in truly majestic terms. Additionally, the speech has a pronounced rhythmic quality that quickly reminds one of poetic verse. Synge's plays, including Riders, are filled with examples of such finely-crafted writing, and for that reason you would be justified in calling Riders a poetic drama. 

Is "Riders To The Sea" a poetic drama?

Certainly!  It's a drama, albeit, only a one-act, and it is poetic.  Synge is at his most evocative in this work, using language to create a world that that is beautiful and awful at the same time.  The beauty is not necessarily aesthetic beauty, but the starkness of the setting and the dreadfulness of the events carry a poetic beauty through Synge's words and phrases.