The tune is a lot older than either Kebon Barry or Shall my Soul- it's the one used for "Rolling Home to Dear Old England" (or to any other place that fits, where you might be rolling home to).

Since Terence McSwiney died on 25th October 1920 in Brixton and Kevin Barry was shot in Mountjoy a week later on 1st November it's not too likely he'd have had anything to do with writing the song.

My impression is that both songs emerged independently, using the same tune.