"She has a girlfriend."
Translation:Tá cailín aici.
I think thats a very literal translation of boy/girl + friend and something I'd use moreso to describe a boy or girl who is a friend rather than someone I'm in a relationship with. Its kind of like the difference between girlfriend (relationship) and girl friend (friendship), but that's my personal interpretation
Irish has a peculiar structure to say have (at least for english natives): to say "X has Y", you say "Tá Y ag X", which literally means "There is Y on X" or, when Y is definite, "Y is on X", so "She has a girlfriend" is "Tá cailín aici" (aici is the "contracted" form of "*ag sí", because almost all preposition get contracted with pronouns in Irish). Hope that helped ;-)