The warnings of the electric fence surrounding the concentration camp are ironic because the fence is certain death to any prisoners who try to surmount it. However, the fence is ironic because it also represents a means of escape and freedom. Furthermore, Wiesel mentions that it's funny they bothered to put up warning signs about the fence when death was lurking around every corner. The ironic sign is basically the warning signs around the fence while Auschwitz literally is a place of death.