You cannot use the subject Chemistry/Physics combined if you're also going to use one or the other by themselves, as obviously that would be unfair. That rule stands for Irish students. HOWEVER presenting Physics and Chemistry as two seperate subjects is fine. They're just protecting from an unfair crossover of subjects.
McKenzie- I don't know whether a D is okay, a D is a pass grade for us but I don't know if it's a pass or fail for you. You need to have passed it anyway. As I said before, it's not stupid. You're competing against people who have studied Irish, English, Maths and French as compulsory subjects for the last two years of school along with 3 others for Leaving Cert, and the same 4 subjects again with 6 others for Junior Cert before that. So it's only fair that you present English, Maths and a language at some level when you're up against people doing those subjects for 5/6 years.