As if to add insult to injury, Denise’s colleagues all detest her for being pretty and learning the job quickly. Zola uses situational and dramatic irony to highlight the conflict between Denise's actual performance and her colleagues' perception of her:
A further torment was that the whole department was against her. To her physical martyrdom was added the surreptitious persecution of her colleagues. Two months of patience and gentleness had not so far disarmed them. [...] Later on, as she quickly became accustomed to the workings of the shop, and proved herself to be a remarkable saleswoman, there was indignant amazement, and from then on the girls conspired never to let her have a good customer. Denise was thus completely abandoned, and they were all utterly hostile to the ‘unkempt girl’, whose life was a perpetual struggle; in spite of her courage it was with the greatest difficulty that she succeeded in keeping her place in the department.
Despite being an excellent salesperson and a quick learner, Denise faces intentional obstruction and sabotage from the other girls who work at the store. This skews her self-perception, making her worry that she is failing in her role. The situational irony here lies in the fact that Denise's competence as a saleswoman is the very reason the other saleswomen converge against her. They recognize her talent and, feeling threatened, go out of their way to ensure she doesn't get good customers. They only allow her to serve people with a reputation for either being nasty or ungenerous. This irony is a commentary on the cutthroat environment of the store: at The Ladies’ Paradise, individual success might make anyone a target.
The passage’s dramatic irony comes from the reader knowing the reality of this confusing situation. The reader understands that Denise's perceived underperformance is a sham, the result of the other girls’ sabotage. They are forced to experience Denise’s pain and worry while she earnestly believes she is not performing well. This creates a tension between the reader's knowledge and Denise's own understanding of her circumstances. She doesn’t know that the real “difficulty” comes from her skill, not her lack of it.