from "Moment with Mary"
Most historians trace the origin of the Rosary as we know it today back to the so-called Dark Ages of ninth-century Ireland. In those days, as is still true today, the 150 Psalms of David were one of the most important forms of monastic prayer. Monks recited or chanted the Psalms day after day as a source of inspiration.
The lay people who lived near a monastery could see the beauty of this devotion, but because very few people outside the monasteries knew how to read in those days, the lay people were unable to adapt this prayer form for their own use. So one day in about the year 800 A.D., an Irish monk suggested to the neighboring lay people that they pray a series of 150 Our Fathers in place of the 150 Psalms. At first, in order to count their 150 Our Fathers, people carried around leather pouches which held 150 pebbles. Soon they switched to ropes with 150 or 50 knots; eventually they began to use strings strung with 50 wooden beads.
In other parts of Europe, the Angelic Salutation, which makes up the first part of our Hail Mary, was recited as a repetitive prayer. Saint Peter Damian (d. 1072) was the first to mention this prayer form. Then during the thirteenth century another prayer form, which would soon give the Rosary its Mysteries, began to develop. Soon Psalters devoted to 150 praises of Mary were also composed. When a Psalter of Marian praises numbered 50 instead of 150 it was commonly called a rosarium, or bouquet of roses.