The historical root of the Rosary can be traced to 290 AD, when the hermits used stones and sticks to count the prayers.
Irish Monks were required to pray the 150 Psalms from the Hebrew Scripture and needed a system of counting and tracking their prayer. They divided the Psalms into groups of 3 with 50 Psalms in each group and prayed it in Latin. The ordinary people not being able to read or know Latin and desiring to pray with the Church, devised a set of prayers based on the 150 count. The 150 Psalms at one point became 150 Paters (Our Fathers) and then changed to 150 Aves (Hail Marys). Eventually other prayers were added. The development of the rosary began with the Scripture and it still continues today to be a Scripturally based prayer.
The first clear historical reference to the rosary is from the life of St. Dominic (died in 1221), the founder of the Dominicans. He preached a form of the rosary in France at the time of the Albigensian heresy. Tradition has it that the Lady herself asked for the practice to counter heresy and sin.