* Caileag mhath: This translates literally to "good girl," but sounds a bit formal and might even come across as slightly patronizing.
* Nighean mhath: This translates to "good girl/daughter" and is a slightly softer option than "caileag mhath." However, the "daughter" aspect might not be appropriate depending on the context.
* A bheil thu math? This translates to "Are you good?" It's a more natural way to express approval, especially for a younger girl.
* Tha thu math! This means "You are good!" - a more affirmative and approving expression.
* Using terms of endearment along with a descriptive word, such as "clever girl" or "sweet girl," might be more appropriate and natural depending on the situation. You would need to provide the context for a more accurate translation.
In short, "Caileag mhath" or "Nighean mhath" are the closest literal translations, but "A bheil thu math?" or "Tha thu math!" are likely better choices depending on the context and your relationship with the person.