A very difficult and interesting topic. Because songs keep changing in the world of folk, "origins" are notoriously hard to pin down. Even a reference to an obvious historic event does not necessarily suggest the original date of a song, as old songs are frequently updated to be of relevance to recent events.
The converse is just as true, antiquarian types love writing songs about the old days (what are often called "fake songs"), and these can confuse the credulous who dont spot the modern style of the lyrics. Plenty of people think Stan Rogers stuff, or the Long Black Veil, or Copper Kettle, or Ellen Vannin. or even obviously modern rock songs like The Night they drove old Dixie down, are old folk songs.
IIt's quite easy to date a genuine lyric that has survived intact from its origins, but folk songs by their nature change in strange, wonderful and unpredictable ways
   Take a trivial example like the reference to "riding the rods" in "The Lakes of Pontchartrain". Does this prove that the song dates from after the invention of railways, and the coining of the phrase "riding the rods"? Of course, it proves no such thing. It merely means someone has at some stage inserted that phrase into the song to make it sound good. That is why they are folk songs! And that is why people try to invent modern ones, with variable degrees of success1