All you'd need is a 555 timer operating at around 40Khz, and connect the send line of the serial port to the 555 timers reset pin (4). Output pin would be 3, going to an IR led. Thus when the serial port outputs 1, you would get 40Khz carrier waves of light.

An IR demodulator at the recieving end will output a "1" everytime it detects the 40Khz carrier.

This is one direction of transfer; build two of these for bidirectional. Just make sure the demodulator and IR led on each side are angled so that they don't interfere. (Stick something between them.)