In what sense was Mary born without original sin in Roman Catholic dogma? Is the following quotation an accurate explanation of Roman Catholic dogma regarding Mary?


  The reason that Mary is “full of grace,” the Church has said, is because of Ephesians 1:3-6 (Redemptoris Mater, no. 7-11). Here is how John Henry Newman puts it:
  
  
    "Does not the objector consider that Eve was created, or born, without original sin? Why does not this shock him? Would he have been inclined to worship Eve in that first estate of hers? Why, then, Mary?
    
    "Does he [the objector] not believe that St. John Baptist had the grace of God—i.e., was regenerated, even before his birth? What do we believe of Mary, but that grace was given her at a still earlier period? All we say is, that grace was given her from the first moment of her existence.
    
    “We do not say that she did not owe her salvation to the death of her son.”"
  


(Source: Catholic Integral Complementarity, by Aimee Byrd)
The most authoritative explanation of the Immaculate Conception comes from Pope Pius IX's dogmatic definition of it in Ineffabilis Deus (1854):


  We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.


The answer to your question of "In what sense was she born without Original Sin?" is:


from "the first instance of her conception."