I can't spell Irish words i haven't seen before the first time i hear them! How was i supposed to know it was mh and not bh? They sound the same! Doesnt that count as a typo?
why isn't it "cailín cheolmhar"?
Because, although it refers to a female person, "cailín" is grammatically masculine, so the adjective is not lenited here.
Yes, the -ín ending on "cailín" makes it a masculine word.
When the -ín is a diminutive ending, it makes a word like cailín masculine. When the -ín isn’t a diminutive ending, the word could be feminine, e.g. braillín.
Then why is it í and not é?
Because that would mean "He is a musical girl".
Grammatical and natural gender don't always agree. In this case you use the pronoun that accurately describes the physical subject, not the grammar.
Cosúil liomsa :D