There’s a joke here that works on a couple of levels, and is made more visible thanks to the adaptation into comics. Dlugacz, the Jewish butcher, is wearing a brown apron. This prompts Bloom to imagine the next-door girl wearing brown scapulars, a sign of a devotee of Our Lady of Mount Carmel, which is the way members of the Carmelite Order refer to the Virgin Mary. Lay people can wear the brown scapulars if it has been blessed by a priest, and it is supposed to grant its wearer eternal salvation. A wearer of the brown scapular may never take it off (although Bloom seems to be suggesting in his daydreams that the next-door girl could be taking other items of clothing off).
So: what we learn here is Bloom may be of Jewish background (through his father), but he also knows enough about Catholicism, thanks to his Irish Catholic mother and living in a city dominated by Catholics and their traditions. He associates the Jewish butcher with a sign of Catholic belief, and he fantasizes about what the next-door girl might look like under her clothes. The brown scapular might be “defending her both ways” (front and behind, thanks to how one wears it over the shoulders), but it’s not defending her from Bloom’s gently leering imagination.