Is it feasible for an engineering student from Germany to get into a Ph.D. program in the UK/Ireland? Specifically, what should one keep in mind/try to accomplish in the last two years before finishing the "Diplom"? How would one apply?
Maybe this question can be extended to other combinations of countries, but I didn’t want it to be too broad.
In short, yes. If you're talking about the "traditional" 5 year Diplom, it's essentially equivalent to an MSc, which is what most universities require to start a PhD (although some only require a Bachelor's degree). I don't think you need to keep anything in particular in mind when finishing, apart from making sure that you get good grades for your transcript :)
The application procedure wouldn't be any different from other students, although you may have to explain in some detail what your degree is.