I agree with dancing piper, above, that "Colleen Due" cannot mean "cailin dubh" because it causes contradictions within the lyric. "Her hair was like the sun-kissed gold" doesn't square with "cailin dubh" which translates as dark-girl or dark-haired girl.

So my bastardized solution is to sing:

No crayon, no chisel, brush nor pen,
Could express my Colleen *true*

Which makes the lyric mean that no artist of any kind could *truly* capture the beauty of the narrator's lost love. It may not be technically correct, but that's what I will continue to sing, as it increases the sentimentality for me. When I listen to Boys of the Lough and allow for their strong Celtic pronunciation, I can almost hear "true."

Hope this helps.

Michael