The song now commonly known as "Wild Mountain Thyme" was written by Francie McPeake of Belfast. McPeake used as a source the Scottish song "The Braes Of Balquhidder," whose version of that stanza is:

Noo the summer's in prime,
Wi' the floores richly blooming,
Wi' the wild mountain thyme
A' the moorland's perfuming

Presumably the non-rhyme in that first stanza originated with McPeake.

According to McPeake's descendants, the most original part of the song was the verse that begins "If my true love she were gone." It was written after McPeake's wife passed away. I got this from Francis III and Francis IV during an interview this past May. I don't know if it's true...but it's their story, and they're sticking to it.