Yes :) no indefinite article in Irish (at least in my knowledge thus far), but we put must put one into a sentence in English; therefore:
cailín = girl -or- a girl (in English, a sentence would not read "Girl drinks." but rather "A girl drinks.".
The Irish definite article is "an" for singular nouns and "na" for plural.
an cailín = the girl
na cailíní = the girls