Try the database under FREECOM - it's The Freedom Come-All-Ye, a magnificent piece by Hamish Henderson (not so old, either)-- the tune is The Bloody Field of Flanders. The Battle of the Somme is an entirely different tune, a 9/8 pipe Retreat, and I don't see how it would fit HH's song at all well. N.B.: the text in the DT has a few misprints, but I suppose you can make them out ok -- "back" for "black", etc., and some others are phonetic, e.g. "geens" for "geans" (with a hard G!)(= wild cherries).