Hi Philip,
IrDa is a little more complicated than just hook up a tx and rx IR led.
The physical layer could be quite simple, but it doesn't use simple
leds, not even the way you know how to modulate a led current. There is
a pulse shape modulation that changes completely the data stream, first
it narrows the pulse, and second, it can only be reconstructed with a
correct receiver microchip. Even that IrDA chips can work in "raw" mode
that is very similar to the asynch way, it still needing the pulse shape
circuits.
Take a look at: http://www.irda.org
Wagner.