Irony is a literary technique that consists of saying the opposite of what you really mean for humorous effect or as a way to emphasize it.
In this case, the sentence is ironic because of the nature of the gift and of the giver. The SS was responsible for the prisoners suffering and were capable of unimaginable cruelty. Therefore, they would not be likely to give them any gift. Secondly, the gift was a restriction that made them uneasy, which is far from "beautiful."
The intention of the author is to use irony to imply the opposite meaning, most likely as a way to emphasize how "horrible" the gift was, because of its opposition to "beautiful."
