The Rosary was already prayed at the time of LePanto. Pius V asked that it be prayed for victory, and of course we know how that went. Afterwords, the Feast of Our Lady of the Rosary was instituted in thanksgiving for God's great mercy. (I attend a Dominican Parish, have for 22 years so the story's been heard several times...)
Thank you for the info. My daily caffeine had not yet hit the bloodstream so I was a bit fuzzy on the details.