"The Minstrel Boy" was an Earth song that Miles O'Brien often used as his "happy thought" in dire situations. (TNG: "The Wounded"; DS9: "Whispers")
Lyrics
The minstrel boy to war is gone
In the ranks of death you’ll find him
His fathers sword he has girded on
And his wild harp slung behind him
"Land of Song" said the warrior bard
"Tho’ all the world betrays thee
One sword at least thy right shall guard
One faithful harp shall praise thee"
The minstrel fell but the foeman's chain
Could not bring his proud soul under
The harp he lov’d ne'er spoke again
For he tore its chords asunder
And said "no chains shall sully thee
Thou soul of love and bravery
Thy songs were made for the pure and free
They shall never sound in slavery"