According to this site ...
"After the Civil War the Republicans maintained their ultimate target: an undivided and independent Ireland. Soldiers of the Legion of the Rearguard was a honorary nickname given to the members of the Irish Republican Army (IRA) by Eamonn de Valera."

Lyrics by: Jack O'Sheehan

And this site of Parliamentary Debates, Dáil Éireann - Volume 1 - 10 April, 1919 says that Mr. Jack O'Sheehan, was sentenced to two years' imprisonment for singing a song. There you go!