what do you know about it hibernicus?
Less than jheem, as it turns out! Cailín is the Irish word for "girl", and as such is not in use as a personal name in Ireland. I did not know, however, the original uninflected form "caile" that jheem mentions.

As it happens, both "caile" and "cailín" are masculine nouns, showing (even better than German "Mädchen", which is neuter) the lack of correspondence between grammatical gender and sex.