There are lots of versions of the song.

Loreena McKinnett did a Two Sisters version on her CD "Mask and Mirror." It lacks the "bonnie broom" chorus, using instead "the swans they swim so bonnie oh." A Twin Cities Irish band took their name, Boiled in Lead, from the song.

Keep in mind that lots of British Isles folksongs have nonsense syllables in the choruses. Some writers (methinks those having an axe to grind) find hints of "ancient pre-Christian beliefs and practices" there.

But is it possible, just possible, that the phrase "lay the bend to the bonnie broom" refers to sweeping the floor, and has NOTHING to do with the narrative?