Thanks Sorcha I know that one 'Chi mi'n Geamhradh' but no not the one I'm looking for. I thought maybe I should put a bit of it up to see if it jogs anyone's memory:

O, s'fhada bhuainn Anna
e ho, rin ho ni do leannan
O, s'fhada bhuainn Anna
Chuala mi gun (thiult?) thu 'n t-armunn
Mac 'ic Bhearnaraigh na Hearadh
O, s'fhada...
Chuala mi gun (thiult) thu 'n t-uasal
a chruachan 's le (????)

Aaargh! it's driving me nuts not being able to work it out. Answers on a postcard please