An interesting combination: Both were originally written in captivity in Germany. The "Hugh Robertson" who wrote "Farewell to the Creeks", I think, is called "James Robertson". He survived the Great War, but I don't know whether he lived to see war break out again (seventy years before now).
There's a thread on Mudcat about The Peat Bog Soldiers, that has quite a bit of information. If you intend to use the English lyrics; nobody seems to know who wrote those. If you're using the same tune as e.g. Pete Seeger, then the tune is out of copyright as it's from the Thirty Years' War. If on the other hand, you're using the original tune, you'll have to take into account that Rudi Goguel, the composer, died in 1976, which means the tune is probably in copyright. Also, if you do have that original tune, please, add it to the thread mentioned above.