The Minstrel Boy was an Earth song that Chief Miles O'Brien often used as his "happy thought" in dire situations. His one-time captain, Benjamin Maxwell, was also fond of the song. A Paradan replicant of O'Brien was programmed to know the song. (TNG: "The Wounded"; DS9: "Whispers")
Lyrics
The minstrel boy to the war is gone
In the ranks of death you’ll find him
His father's sword he hath 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 rights shall guard
One faithful harp shall praise thee"
The minstrel fell, but the foeman's chain
Could not bring that 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"
Background
The Minstrel Boy is believed to have been written by Irish poet Thomas Moore (1779-1852) to commemorate friends who had died in the 1798 Irish rebellion against British rule. The song has been used in many film and television productions including Black Hawk Down which starred Tom Hardy (Star Trek: Nemesis Shinzon) and Ronald D. Moore's Battlestar Galactica.
The tune of The Minstrel Boy is played in the soundtrack of Star Trek: Deep Space Nine's final episode "What You Leave Behind". It can be heard playing during the scene where O'Brien takes one last look at his quarters and finds the figurine of William B. Travis.