The beautiful score for the opening credits for Outlander were composed by Bear McCreary (who also composes all music for the show) and performed by Raya Yarbrough.
In season one, the theme featured strictly Scottish elements while during the first half of season two, when Outlander was in Paris, the music highlighted French music and lyrics.
Sing me a song of a lass that is gone,
Say, could that lass be I?
Merry of soul she sailed on a day
Over the sea to Skye.
Billow and breeze, islands and seas,
Mountains of rain and sun,
All that was good, all that was fair,
All that was me is gone.
The song tells how Bonnie Prince Charlie, disguised as a serving maid, escaped in a small boat after the defeat of his Jacobite rising of 1745, with the aid of Flora MacDonald. The song is a traditional expression of Jacobitism and its story has also entered Scotland as a national legend. (Wikipedia)
The original lyrics were changed from lad to lass to fit the show.