Liland
I think Liz may be right that the search will be a hard one for the reasons given below. However there are now quite a few Cornish language classes going on and we have one family come to our folk club whose first language at home is Cornish, kids an' all.
We have Cornish speakers in my village and I am regularly greated with "myten dha, fatla genes?" instead of "good morning, how are you?"
The song was first written down about 90 years ago, but is probably about 60 years older than that. It comes from the village of St.Day in Cornwall.
Helston is my nearest town and the guy that runs it (who's name escapes me) is a Cornish speaker. I will ask him about it and look through the books in the shop.
As Liz mentions the last person to speak Cornish as a first language was Dolly Pentreath who was interviewed by Daines Barrington in 1768.
Merv Davey in his book Henegan says that there are probably only 5 traditional songs that survive written in Cornish. If you are interested as to what they are let me know.
If a translation from English to Cornish will suffice let me know I and can sort that out.

KJ