Yes, Lizzie Rainey and David Brown married on 24 Dec 1903. My Grandfather Robert was born in 1906.
I think the protestant/catholic barrier really formed after 1912... I still think it is likely they went to the same school. Lisburn would be traditionally a protestant town but your grandparents must not have been too bothered with the British/Irish debate as he signed up to fight in the British army and your great grandfather was in the RIC.