If you are specifically looking to integrate EagleIron with a user-defined Fortran subroutine, the workflow involves compiling the Fortran code into a shared library ( .dll on Windows, .so on Linux) and calling it via P/Invoke.
: Eaglercraft is often packaged into a single, small HTML file that runs locally. This makes it highly portable and useful in environments where gaming sites are restricted. System Requirements for 1.19