Maith an cailín
Good girl, in Irish. Used in English as praise for a girl or woman who has done well. For a lad it's maith an buachaill.
Ye passed yer test first time? Maith an cailín!

Shall I translate it into posh English for you, boi?
Good gracious, a reader! How thrilling. Here it is in posh English.
You passed your driving examination at the first attempt? Well done, my girl!
Questions
What does maith an cailín mean in Ireland?
Good girl, in Irish. Used in English as praise for a girl or woman who has done well. For a lad it's maith an buachaill.
How do you use maith an cailín in a sentence?
“Ye passed yer test first time? Maith an cailín!”
Updated
On the list: Irish slang phrases