Linguistically, from the use of "y", it would seem that "ultreya" is the galego version of the latin "ultreia".
For reference, note that "Santiago" in galego is "Sant Yago", while in Shakespeare's Othello [set in Venice and Cyprus...much closer to Italy and its Latin] it's "Iago" not "Yago"...
Either way, you certainly don't see it written as often as you hear it spoken along the camino!