here us another 20th century song about a North Uistman off to war. Calum Mac an Tòisich wrote the words of 'S Fhada Bhuamsa Thar a' Chuain from his own experience of WW2, and Catríona Garbutt set the words to music. http://www.bbc.co.uk/alba/oran/orain/s_fhada_bhuamsa_thar_a_chuain/
That recording isn't so much to my taste, but I love most of Julie Fowlis' work. She herself has composed a tune for a world war 1 Gaelic poem,"air an Somme" by Domhnall Ruadh Corona,