The idea of the priesthood of Mary
"seems to have been around since the writing of the early Fathers of the Church in the 4th century and is wonderfully illustrated in striking mages of Mary dressed in priest’s vestments – some dating from as early as the 6th century."
"First of all I would like to refer to the question of the title of priest attributed to the Virgin in tradition. A writer of the end of the fifth century calls Mary "Virgin, and at the same time priest, and altar who has given us Christ -- bread of Heaven for the remission of sins."1 After this, there were frequent references to the topic of Mary as priest, which subsequently became the object of theological developments in the 17th century, in the French school of St. Sulpice. In it, Mary's priesthood is not placed so much in the context of a relationship with the ministerial priesthood, but rather with that of Christ."- Fr. Raniero Cantalamessa, OFM Cap, Pontifical Household Preacher.
"In the seventeenth century, Salazar and other Spanish theologians compare her role with that of the Christ and identify it with her redemptive mission.9 In the same century, another line of thought associated with Berulle and the French School of Spirituality appeared, and continued throughout the eighteenth century. While its inspiration originates in the Spanish school, this new line of thought overshadows Salazar's interpretation, due principally to Olier and the seminary of St. Sulpice. In the French School, Mary was invoked and contemplated as the model of the priest, and honored as "Virgo Sacerdos, the Virgin Priest."10
In the nineteenth century, the Marian writings of the early decades are vacuous and sentimental. By the middle of the century, a rebirth is detected. Theologians began to restore to Mariology its theological content, and to connect again with the movements of the seventeenth century. Once again, mediation, co-redemption and the sacerdotal aspect of Mary's mission gain ascendancy in their studies.11
Around 1870, the idea of living as a victim began to gain popularity among a number of generous souls, especially women religious, who proposed to assist the priests through their prayers and sacrifices. They thought naturally of Mary praying and offering herself for and with her son, and they loved to consider her as their sacerdotal virgin or the virgin Priest. This devotion aroused great enthusiasm, and was at times expressed in formulas scarcely theological.12"- Br. John M Samaha, SM
"In 1906, Pope Pius X approved a prayer which says "Mary, Virgin Priest, pray for us." (emphasis mine). However, a Holy Office decree of 1916 forbade the use of any image that had Mary wearing vestments – some say for fear of the possibilty of an argument for women’s ordination, others that Mary as a priest was a metaphorical image taken too far."-The Priesthood of Mary
"During the reign of Pius X, the Holy Office issued a decree stating that "the representation of Mary clothed in sacerdotal vestments was disapproved." In reality, the representation in question was that of an orante, which some persons mistook for Mary vested as a priest. In 1926-1927, the Holy Office again opposed the propagation of devotion to the "Virgin Priest." Even though only the picture and the spread of this devotion have been forbidden, Rome is evidently unfavorable to this title, since it might lead poorly-instructed Catholics to believe that Mary had received the sacrament of Holy Orders. Yet these decrees of the Holy Office in no way affected the pronouncements of Popes Pius IX and Pius X that Mary was "an associate of the Divine Sacrifice," and that she was enriched with "as much dignity and grace as are found in the priesthood."13 - Br John M Samaha, SM
If Mary is a priest, it is not by virtue of priestly ordination because she was not ordained a priest, but rather by virtue of her association with her Divine Son during the Passion.
Images of Mary wearing priestly vestments are now forbidden. We may have a personal devotion to Mary under the title of Virgin Priest, but may not spread its devotion without the approval of Rome!