There's no TIR support in FTNOIR because of Windows requirement for signed drivers, because the company manufacturing the device is known to be litigious, and finally, because someone would have to do the work.
There's a TIR driver in the linux-track project however, but only for Linux and Mac.
If you're also speaking of NPClient reverse-engineered sources, they're kept private, to help avoid legal harassment by the aforementioned company.
FreeTrack can actually work on Linux, with use of the 'ftnoir_compat' library in the posix branch. It provides a uniform way to access mutexes and shared memory mappings between platforms. After that, all that's required is changing windows.h non-standard stuffies to their ANSI/POSIX equivalents, maybe also some calling convention preprocessor macros and some other implementation details.
PS Linux support most likely won't make it into FaceTrackNoIR due to original author's objections, therefore we've forked the project as
https://github.com/opentrack/opentrack