Well, I'd have to say it would have to be 'An Cailin Rua' (The Red-haired Girl) -- How could I miss with that song!

Translation of first verse: Isn't it hard for me to praise my red-haired girl
Not only because of her red hair
She was like a ray of sun reflected through glass
And my red-haired girl had the beauty of the women of the Finne

-- Áine