They sing a lot about workers rights and unions, maybe you are joking. "Boys on the Docks" is almost what you're after, but not saying directly to join a union.

Ken: It’s brand new. It’s about my grandfather, John Kelley. He organized most of the union workers down on the fish pier.

Reference: https://lollipopmagazine.com/1997/10/dropkick-murphys-boys-on-the-docks-interview/

Also, their song "Which Side Are You On" starts with the lyrics:

Our father was a union man, Some day I'll be one too.