Breathnach doesn't actually draw a "conclusion" - I just looked the book up. "...is said to have been written..." is as far as he goes. Munnelly makes it look like Breathnach was reporting it as fact.

Breathnach has a paragraph about "Callino casturame" in his book, with notation (a transcription from William Ballet's lute book). He says the rediscovery of this tune was due to the Irish Shakespearean scholar Malone, in the 18th century.