This problem is really not that bad at all. You do not need to send the carrier frequency over the headphone jack. Instead, you should build the oscillator into your device a-la http://jap.hu/electronic/infrared.html (or just use a micro) and then you just need to switch it on and off based on a much slower signal from the headphone jack.
According to the internets, almost all remote control receivers expect a 38kHz carrier wave. You will need to turn it on and off at 2-4kHz. See also http://en.wikipedia.org/wiki/RC-5
If you are as clever as off-the-shelf iPhone headphone jack infrared transmitters then you can get your power from the audio signal, but if I were you I'd just use batteries.
well, in general, you should have right, left, and mic, with a ground for each on the iphone end, and all 3 will have a common ground. for the headphones, right and left, with a ground for each, and ground is shared.
you will want to remove the protective coating with a microtorch or other heat source, solder and heatshrink, as opposed to twist together. You will also want to look up the pinout for the iphone side of it, and use a continuity tester to identify what each cable usually is.
red is USUALLY right, blue or green is is usually left - the white on the sennheiser is unusual (i double check this either by following the cable to the end, or using a temporary connection with crocodile clips between a prepared, tinned end of the headphone cable and jack).
Red to red, white to green, and 'copper' to 'copper' should do the trick. all 3 have the pesky coating, so you'll need to remove it.
this is the exact process/tools that have worked for me in the past - but instead of the jack, tin both ends, press to check, then solder.
Best Answer
I'm not that great with Android App development so I'm assuming you can make the headphone jack output a voltage in reference to the "common" ring. This guy got his android to output a waveform that was dependent on the accelerometer device readings.
Most smartphones have TRRS jacks on them, where the breakdown looks like this:
So if you can control the voltage difference between "Left" or "Right" and "Common", then you could exploit that.
Have the voltage difference feed into a transistor, while your external battery drives your LED. This is a much better method because you don't really want your phone to drive your LED, you just want it to say "OK, turn on, now turn off" etc.
simulate this circuit – Schematic created using CircuitLab
So it's just a really simple circuit, where the phone activates/deactivates the transistor. When the voltage of "LEFT or RIGHT" goes HIGH, current will travel from the battery, through the transistor and through the LED. Setting the voltage to ground will stop any current from flowing.