The author says Jack wore something "like his old clothes". What literary device is used and what is the irony of this?

The phrase "something like his old clothes" is a simile, a comparison using the words "like" or "as". The irony is that he is wearing what used to be his old clothes, but they have become so tattered and torn with time spent on the island, that they no longer look the same. Jack has begun to look more like a savage than a choir boy.