I'd like to offer the following chord sequence. I got it from "The Folksingers Wordbook", and to my ear it's a better fit to the tune than the one previously listed.

          Em                 Em              G       G  
With your guns and drums and drums and guns, harroo, haroo
  
          Em                 Em              G       B7  
With your guns and drums and drums and guns, harroo, harroo
  
          Em                 Am                  Em           B7  
With your guns and drums and drums and guns, the enemy nearly slew ya
  
   Em      Am       Em      B7     Em       Am     Em  
My darling dear you look so queer, Johnny I hardly knew ya