Simply insert the missing 'w' into Jack's link.

A scan of a setting for voice and piano can be found in pdf format at http://www.seanet.com/~inisglas/images/ancronanbaissongsheets.pdf.

No information is given as to where it was copied from. Songs of the Hebrides, presumably. Certainly not Carmina Gadelica.

I'd take anything you find on those neo-pagan 'Celtic' websites with a large pinch of salt, though; particularly those that don't tell you how they know all the things they announce as 'fact'. MacLeod himself cobbled the text together from bits and pieces ('various fragments are pieced together here', he wrote); whether any of it genuinely belongs to the tune I don't know. There is a chapter devoted to it in MacLeod's The Road to the Isles: Poetry, Lore and Tradition of the Hebrides (Edinburgh: Grant & Murray, 1927, 127-134, 'The Death Croon'): very romantic, but not very informative.