The script states, "He sings the chorus of an old love ballad in either Welsh or Gaelic." The caveat follows, stating, "Selection of ballad must be left to Mr. Doohan since this writer knows nothing of either Gaelic or Welsh." The closed captioning for "The Lorelei Signal" simply overlays the text "Scotty singing melancholically."
The Star Trek Concordance states that this is a "Welsh ballad". It also says that Scotty sings this song again in "Mudd's Passion". However, this is either an error or he is singing a different part of the song that sounds nothing like this part.