A chairde,

The meaning of "cad é sin don té sin nach mbaineann sin dó" is: "what's that to the person that it doesn't concern."

If you're not going to sing the song in it's native language, why bother? The words and tune were crafted together in Gaeilge, not in English, and more is lost than just the 'meaning' of the words when it is performed in English. If you're going to perform this song in front of an English-speaking audience, just give a general explanation of the background and meaning or 'feel' of the song and sing it in Irish. Give the people some credit – they'll be able to 'get' the song without knowing what every single word means.

Le meas, Áine