What is ironic about Bruno thinking it is unfair that Shmuel has many friends while he has none?

Irony is when the opposite of what is expected happens. Bruno doesn't understand that while Shmuel may have other people in the camp with him, they are not happy. They do not get to do the things that friends do, and they aren't completely friends. Bruno regards Shmuel's situation as better and more exciting than his. In actuality, Bruno has more freedom and a better life than Shmuel.

This entire scene shows Bruno's ignorance of what is happening on the other side of the fence.