The easiest option is to use Conda, as STIR is on conda-forge, see our wiki page.
You could also use the SyneRBI VM, or its docker image (currently this only has the STIR Python interface, but not the STIR executables).
You can also build STIR yourself using CMake. Please consult the STIR User's Guide (available in the documentation and on the web-site) and the STIR Wiki.