Using interrrupts with RC522
From your code:

void isr()
{
  Serial.println(F("Interrupt"));
  mfrc522.PCD_WriteRegister(MFRC522::ComIrqReg, 0x80); //Clear interrupts
}


Do not do serial prints inside an ISR! They will eventually hang it. What you need to do is have a volatile variable (eg. a bool), set that in the ISR, and then check that in loop. If it changes, display it. Eg.

volatile bool cardPresent;

void loop() 
{
  if (cardPresent)
    {
    Serial.println(F("Interrupt"));
    mfrc522.PCD_WriteRegister(MFRC522::ComIrqReg, 0x80); //Clear interrupts
    cardPresent = false;
    }

}

void isr()
{
  cardPresent = true;
}




The other thing I would do is put a scope or logic analyzer on the IRQ pin and see if it is actually changing. It should trigger the interrupt if it does.