What would be the possible theme the poet thinks about the reaper's song?
A general all-encompassing theme for both the reaper's song and the poet's feelings might be classed under 'remembrance.' It is interesting for us too to remember that the poet wishes he could stay for longer, forever even, to carry on listening to the allure of the reaper's song - and that it is the sort of spectral music or mood that haunts the listener long after he or she has left the presence of the singer. The reaper's own remembrances can only be guessed at as the poet cannot get anyone to translate the Gaelic language for him - this language is spoken in differing forms in Ireland, Wales and Cornwall as well. Irish Gaelic music in particular is known for its 'mournful airs' or 'sorrowful tunes.' Today we use music as a remembrance in such rituals as the playing of 'The Last Post' where the sad notes of the bugler have us remember 'The Fallen.'
The poet was not sure about the theme of the girl's song because she was singing in 'Gaelic', a native language of Scotland. But from the sad tune of the song he guessed that the girl was perhaps singing about battles fought long ago or some sorrowful incidents of the past or about some commonplace matters of sorrow that happen in our everyday life.