Hello, Guest from China

There's more than one set of words sung to this tune, and that makes things complicated. Some are about a failed love affair and some are about soldiers. The tune is often called by its Irish name, Shule Aroon or Suil a Ruin (same thing spelled differently)

This thread that we are on is called 'Gone the Rainbow,' so I will explain that particular version.
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A mother is sitting on a hill called Buttermilk Hill, and she is singing to her baby. The baby's father has left them to become a soldier. The mother sold her rock, her reel and her spinning wheel (these are things she needs to make thread from fibers) to make money to buy a sword for the baby's father.

In the last verse the mother sings 'gone the rainbow, gone the dove.'
These are symbols that mean that the beauty and peace have gone from her life.
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In the 1960's (or maybe 1970's) a trio of singers called Peter, Paul and Mary (PPM for short) recorded this version, which many people heard on the radio and still remember. The song wasn't really long enough, so it was padded out with nonsense syllables.

A long time ago, the nonsense syllables may actually have been words in the Irish language, but nobody knew about that in the 1960's.