The fact that a song was collected from an illiterate in the late 20th century is no evidence at all that it was handed down orally for centuries.
Who's talking about the late 20th Century Jack

You were. You were describing songs you'd collected.


the oral versions of these songs date back a century or so earlier than that.

You can only know that if somebody wrote them down - if they stopped being purely oral at that moment.


The ones I'm working on now date back to the years following the Irish famine - Burns was collecting songs form Scots peasants earlier than that

And he published them. In books which sold in enormous numbers and got to the remotest corners of the English-speaking world within 10 years of his death. So everybody everywhere learned his versions. Found a version of "A Red Red Rose" that doesn't derive from his?


Up to the middle of the 19th century literacy was an largely an urban phenomenon and the percentage of people who could read was fairly small

It didn't take many. And we know there were enough to provide a living for itinerant chapbook sellers.


I find the idea of a farmworker going along to the local literate and asking him to interpret the printed words of a song rather.... well!

I can't help your lack of imagination. Anybody who could read the Bible could read a chapbook (though perhaps not all of them would want to). That's not a bunch of elite experts.