How to modulate Transceiver module for use with ATmega164PA
IRDA 'modulates' RS232 by converting it to/from RZI (Return to Zero Inverted). An ATmega164 running at 8MHz should be fast enough to 'bit-bang' this format on standard GPIO pins, but timing is critical so it won't be able to do much else at the same time. 

You could use AVR304 (http://www.atmel.com/webdoc/atmel.docs/atmel.docs.33085.18343.html) as a starting point for your own code. Modify the Transmit code to generates a short pulse for each '0' bit, and during Receive if a pulse is detected then the bit is a '0' ('1' bits are represented by no pulse occurring during the send or receive bit time).

It might be possible to use hardware PWM for generating the transmit pulses, but I would try software timing first because it's easier (you can try other techniques once you get it working!).