A few more notes

Many of Ewan McColl's songs are often mistaken for traditional - just shows his mastery of the idiom.

Happy Birthday to You is NOT traditional, it is still in copyright

My Johnny was a Shoemaker - composed in America c1860, author known.

Wild Mountain Thyme/Braes o'Balquhither (I think that's the original spelling) by Robert Tannahill c1790's

Danny Boy - words written 1911 by an ENGLISH lyricist

In fact, versions of the majority of 'folk' songs can be found on printed broadsides from the 1800's or earlier (some as long ago as the 1600's). That doesn't mean that they are NOT traditional, but sometimes the author is given. For example, the author of the original of Wild Rover (in the 1670's, I think) is known.