I've never seen this as a girl's name and to be honest I think it works on a boy much better. It's a really nice name; handsome and nice-sounding though very Welsh.
I love the name Emlyn (for a boy of course) but it has been ruined by the horrible ultra popular name Emily. Sad. And Emlyn on a girl looks like someone was obsessed with the name Emily but said "hey lets make her different and call her Emlyn yay." That's stupid.