In this case, the song was originally written with an accompaniment.  Copies scored for voice and piano may be seen at  The Lester Levy Sheetmusic Collection.  Mind you, the original song is very different from the version that people sing nowadays; it got taken up by the Copper family of Rottingdean in Sussex, and in their hands a nice, but unexceptional 19th century parlour song turned into something quite different and far more impressive.  I've never heard a recording of the original song; so far as I can tell, everybody who sings it nowadays is singing the song as remade by the Coppers, whether they realise it or not, though people like Mary Black (who learnt it from her brother, who learnt it from Nic Jones, who learnt it from the Coppers) have tended to regularise the rhythm of the song to the point where it's in danger of turning into a Country & Western piece.  While generally I avoid threads where somebody wants "the chords" to a traditional song (which chords? which version? what instrument? why can't people work out their own if they feel the need to accompany a song that was almost certainly made with no accompaniment in mind, etc...), in this particular case there really are original "chords" available.  Of course, they won't fit the song as it's sung now...

Malcolm