In '85 I planned to join the RM and attended the recruiting office in Holborn, London. The RM NCO asked me about my family background and on hearing that he was an Irish labourer he asked about what my Dad did. I told him that my Mum and Dad were separated and I didn't know what he was up to.
Quick as a flash he turned down my application, but advised me to join the RN instead - still here today!
i would imagine the issue with irish nationalities is the conflict over there.
the RM dont want to be recruiting people who then have to go back home and point guns at their friends.
not to mention the possibility of members of paramilitary groups joining up and turning a blind eye to allow fellow members to cross checkpoints with weapons etc...