The words "I love you" appear in "Don't be foolish, pray" -

"Now Molly, while I love you so,
Why still our joys delay?"

(That's from the printed source. Nic Jones, for whatever reason, sang "Why still our hearts deny?")

An actual full-on direct-speech "I love you" is going to be harder to find, as it implies the singer's addressing his beloved. Maybe a song of parting?