interface IrDA with encoder
To trouble shoot i would hook up the TXIR RXIR(mcp2120) pins together and make a loop back device. Just to make sure your encoder/decoder works, what you send to the Chip should be echoed back to the uC. Once you confirm that the TX RX lines are working, you would then test to see if your IR sensors are transmitting and receiving by using a digital camera or webcam. The other way to debug this is if you have an oscilloscope on hand to check the data lines.

all your connections and pins do look correct, and your clock source is correct for your baud pins for 9600