Theologians have traced the origin of the Rosary back to the Ninth century, and a form of prayer that evolved in the monasteries of the early Irish church. Prayer and labor filled the days of the Irish monks, and one of the most important forms of monastic prayer was the daily chanting of the 150 psalms of David. Lay people around the monastery would hear the psalms every day as they were sung or recited, and the beauty of this form of prayer intrigued them. They yearned to join in, but the psalms were too long to memorize, copies could not be found since printing was rare, and few knew how to read Latin anyway. The lay people were however, determined to adapt this prayer form for their own use.
Sometime around 800 AD, the people's desire to participate led to their reciting The Lord's Prayer in response to every psalm recited by the monks. As this form of devotion became popular, people began to carry leather pouches of 150 pebbles, in order that they might keep count of their daily prayers when they were not in hearing distance of the monastery. A thin rope with 150 knots became less of a burden and soon replaced the bag of stones. The Celtic infatuation with the number three, soon saw the prayer rope evolve into a rope of 50 knots to be said three times, and this became an accepted standard.
When the Irish missionary monks began to travel and evangelize Europe, this form of devotion was brought with them. In some areas, clergy and lay people began to recite the Angelic Salutation which makes up the first part of the Hail Mary in response to the psalms. St. Peter Damien, who died in 1072, was the first to mention this form of prayer, the popularity of which led to the daily recitation of 50 Angelic Salutations on a knotted or beaded prayer string.
During the 13th century, the recitation evolved into yet another form. Medieval theologians began to interpret the 150 psalms as veiled prophesies about the life, death, and resurrection of Jesus, and they composed a series of psalters, or praises, based on each interpretation. Soon 150 psalters in honor of Mary were also composed. In order to fit the existing prayer string, the psalters were divided into three "rosariums" or bouquets of 50 each. This was the form that St. Dominic knew, and promoted.
With the Church's emphasis on unity, it was inevitable that a planned combination of all the prayer forms was prescribed as a standard. The first step toward that standard took place about 1365 when Henry of Kalkar, Visitator of the Carthusian Order, divided the 150 salutations into decades of 10, with an Our Father preceding each. Around 1409, another Carthusian named Dominic the Prussian, wrote a book which attached a psalter of 50 thoughts, about the lives of Jesus and Mary, to a Rosarium of 50 Hail Marys. The division of the 50 Hail Marys into five groups of ten, or decades, with an Our Father before each, gave the modern Rosary its form, yet the evolution was not over.
To say that the Crusaders introduced the Rosary to the Catholic faith appears to be inaccurate, though. I can't imagine why anyone would be interested in making such a claim, can you?