What is the oldest Marian hymn?

On this fragment, in Greek, is inscribed a hymn to Mary, called in Latin Sub Tuum Praesidium, or “Under Thy Protection.” It was part of the Christmas liturgy of the Coptic Church, and continues to be sung in both the Eastern and Western Church today.
The fragment is from around 250 A.D., making Sub Tuum Praesidium the oldest known prayer to Our Lady. Obviously this is just the date of the fragment itself, so the hymn may be older, and there are likely other ancient devotions that have not made it to subsequent generations.
Additionally, the lyrics hail Our Lady as “Theotokos”—“God-bearer” or “Mother of God”—showing that this title was already applied to her long before she officially received it at the Council of Ephesus in 431 A.D.
We can see, therefore, that devotion to Our Lady dates back to the very earliest centuries of the Church.
Here is an English translation of this beautiful song:
Under your mercy we take refuge,
Mother of God!
Do not despise our prayers in necessities,
but from danger deliver us,
only pure, only blessed one.