Inside the tar are the 3 following directories:
mcubin
usbirboykmod
usbirboymcu
mcubin contains a single hex file that is already compiled to byte (hex) code via codewarrior (a development env for Windows). This hex file is what the chip needs to be flashed with. If you want to compile it yourself, the source code in usbirboykmod is what you will need. From reading the README file here, it seems that compiling this firmware under sdcc renders it useless so if you want to compile it yourself you will have to do so in CodeWarrior. Now for the stupid part.
usbirboymcu contains all the source code files for the driver. The README explains that this driver also needs to be compiled under CodeWarrior. Sadly enough it is only available for windows so a little work around envolving wine and codewarrior is needed. Apparantly, this driver source can be compiled under sdcc so this may be a better possibility than using an emulated env to run codewarrior.
The README file in this driver source directory gives pretty decent directions on flashing the chip with a prommer (not provided). This of course will be the most tedious task if you are not familiar with, or do not have a hardware pic flashing device (prommer). I think you can find cheap ones around the net. This project seems to provide a schematic for building a prommer to flash the MCU with. This may be YOUR best bet since you are already an established hardware hacker

Hope this helps some. I really only re-iterated what the README's stated along with a little more elaboration on some of the terminology and methods required to accomplish this.