If I where you i should have used a Power MOSFET transistor, for example BUZ10, and put a resistor about 20 Ohms in serial with the IR diode.

The transistor you are using now, i dont think it can handle more than 100mA
and an 8051 has only pullup resistor to drive a port pin high wich maybe isnt sufficient to drive a standard NPN transistor, but surely a MOSFET gate