Friendly coding suggestions:

Here are some comments I have to make while briefly going through the code.

1. ROOT's TObject is not really needed anymore.
2. Avoid using ROOT specific-stuff unless it is really needed. Example from my experience, TRef is usually more of a headache than helpful.
3. From my other issue about using #include quotes : https://stackoverflow.com/a/3162067/13377601
4. Avoid unnecessary inheritance. Example: TObject -> TransientParticle -> {ChargedParticle, OpticalPhoton}. I do not see any polymorphism in the interface so there is no need for inheritance at all here.
5. The code is hard to read with all the pre-processor defines and logic.
6. The IRT::Solve member function should (ideally) be const.