The "scapulars" mentioned in Calypso, Nausicaa, and Circe are Catholic garments that hang from the shoulders (the name derives from the scapula or shoulder blade). Many monks and nuns wear long, shoulder-width, apron-like outer garments called scapulars as part of their habit, but the ones Joyce has in mind are smaller devotional items worn by laypeople. They are usually small rectangles (no more than a few inches on a side) connected by two cords or cloth tapes so that one patch can lie on the chest and the other on the back. Most often, they signify devotion to the Blessed Virgin Mary.
The servant girl that Bloom sees at the butcher shop is wearing "Brown scapulars in tatters, defending her both ways." He is imagining that she wears the Marian devotional images to safeguard her virginity, though this is a crude reduction of the religious significance of the practice.
Brown scapulars (there are various colors) derive from Carmelite devotion to Our Lady of Mount Carmel. Kitty-Kate makes clear in Circe that they are the badges of a sodality of Catholic women. In her case the defense of virginity did not work: "I forgot myself. In a weak moment I erred and did what I did on Constitution hill. I was confirmed by the bishop and enrolled in the brown scapular. My mother's sister married a Montmorency. It was a working plumber was my ruination when I was pure."
Despite this continuing fixation on the supposed anti-sexual powers of scapulars, Bloom does know that they have a broader protective function. In Nausicaa he thinks of sailors who go off to sea: "Off he sails with a scapular or a medal on him for luck."