They are probably both following the path set by Woody Guthrie, Pete Seegar and others. I think that Andy's been writing since at least the early 70s.

Check out Andy's work with Sweeney's Men, Planxty, Patrick Street etc, but if you like his songwriting (who wouldn't) get 'Rain on the roof', 'Rude awakening' and 'Way out yonder'. Andy's main subjects include working-class heroes and explorers. No one writes 'em better.