How to send unmodulated IR-Signal via cable?
There are two major components to an infra-red signal.  One is the high frequency carrier wave signal that pulses the LED on and off.  This frequency has to match that of the receiver and is used with either a high-pass or band-pass filter to filter out ambient IR light and allow it to only react to a real signal.

The second component, and it sounds like this is the one you are interested in, is the on-off pulsing of that carrier wave in different combinations and with different timings to denote binary 0 and 1.

Without knowing what your cable actually is, and we'd need to see a datasheet to get all the proper information, all this is just surmising.

I would imagine that you provide power and ground to the emitter cable, and the third pin is a logical "on/off" signal that enables and disables the carrier frequency to the LED.  That would allow you to send any codes you like as long as you know the timing of the signal.

In the library you link to is all the timing you need in the ir_NEC.cpp file.  The times are defined as:

#define NEC_BITS          32
#define NEC_HDR_MARK    9000
#define NEC_HDR_SPACE   4500
#define NEC_BIT_MARK     560
#define NEC_ONE_SPACE   1690
#define NEC_ZERO_SPACE   560
#define NEC_RPT_SPACE   2250


The actual data format is defined in the sendNEC function:

void  IRsend::sendNEC (unsigned long data,  int nbits)
{
    // Set IR carrier frequency
    enableIROut(38);

    // Header
    mark(NEC_HDR_MARK);
    space(NEC_HDR_SPACE);

    // Data
    for (unsigned long  mask = 1UL &lt;&lt; (nbits - 1);  mask;  mask &gt;&gt;= 1) {
        if (data &amp; mask) {
            mark(NEC_BIT_MARK);
            space(NEC_ONE_SPACE);
        } else {
            mark(NEC_BIT_MARK);
            space(NEC_ZERO_SPACE);
        }
    }

    // Footer
    mark(NEC_BIT_MARK);
    space(0);  // Always end with the LED off
}


The mark and space functions take a number of microseconds to perform either an ON or OFF of the PWM signal.  That is somewhat simpler for you, since it is just an ON or OFF of a digital IO pin to control the output of the emitter cable.  So where you have a MARK you turn on the output, then delay for the specified time. For a space you turn off the output and again delay for the specified time. 

So you can just replicate the operation of that sendNEC function but with your own output control and timing in it instead of using the functions in that library.



Edit: It seems the cable doesn't need power - just provide the ground connection and the signal connection.  Of course, this is assuming that it isn't just an LED on a wire.  If it is just an LED on a wire then you would need to just use the library you link to as it is.