In the Lemon Jelly song 'All the Ducks' what does "Fal de ral de ral do" mean?

Anyone Dutch out there?

I am still working on the translation but it seems that the version in your Q might be a verbatim rendition of an old Dutch folk song.

All ducks swimming in the water

Text

Alle eendjes zwemmen in het water All ducks swimming in the water
Falderal deriere, falderal de rare Falderal deriere, falderal the rare
Alle eendjes zwemmen in het water All ducks swimming in the water
Fal de ral de ral de ral de ra Fal the ral the ral the ral the ra
Alle eendjes zwemmen in het water All ducks swimming in the water
Falderal deriere, falderal de rare Falderal deriere, falderal the rare
Alle eendjes zwemmen in het water All ducks swimming in the water
Fal de ral de ral de ral de ra Fal the ral the ral the ral the ra


Quite possibly it is a simple refrain with no meaning at all.

http://translate.google.ca/translate?hl=en&sl=nl&u=http://www.geerts.com/dutch-songs.htm&sa=X&oi=translate&resnum=7&ct=result&prev=/search%3Fq%3Ddutch%2BFal%2Bde%2Bral%2Bde%2Bral%2Bdo%26hl%3Den

The song's real title is "Nice Weather for Ducks."

As for the lyric 'Fal de ral de ral do,' I can only find the reference to 'Fal de ral': Derogatory term for the nonsensical business and waste of Christmas. http://www.urbandictionary.com/define.php?term=fal%20de%20ral

But I really don't think it as any real meaning.

These are all traditional English words just for singing, like bebop and also Bhangra which all have standard words simply used for rhythm with no actual meaning, but yours is one of the oldest from the days of travelling minstrels.

I think mscorpion is on to something:

Falderol -- fal·de·ral 1. mere nonsense; foolish talk or ideas. 2. a trifle; gimcrack; gew-gaw.

http://dictionary.reference.com/browse/falderol?r=14

What better to accompany a bunch of swimming ducks?

I don't know this song, and I don't know much Dutch.
But I do know that we have a similar expression in Norwegian song texts, "falle ralle ra", which basically means "tra la la", or, if you wish, nothing.

Why fal de ral, I don't know. Whether it was originally a word for nonsense, or if that came after the fact, it was often used as a refrain in folk songs, as Dave said, so everyone could sing along without having to be familiar with the tune or know all the lyrics. Variations abound, like tra la la, fa la la, doo-be-doo, or any short, easy to remember phrase that is repeated throughout the song. A more modern example would be Doo-wop tunes which usually incorporate a nonsensical "shoo-be-doo" or "be-bop-de-wop" that everyone can sing along with.
https://en.wikipedia.org/wiki/Refrain

Speaking from a Dutch point of view: all those answers are correct.

"Alle eendjes zwemmen in het water" is an old children's song.
(for the karaoke version listen here: http://www.liedjesland.com/Liedjes/kinderliedjes/Alle_eendjes_zwemmen_in_het_water/Alle_eendjes_zwemmen_in_het_water.htm)
"Fal de ral" etc. has no meaning.

A reason for the use of nonsense syllables in songs is to enable the audience to join in. Lines like 'fol de rol de riddle ol, fol de rol de day' are easily remembered by the lowest level of listener and give a sense of participation to people that couldn't get up and perform themselves. The use of apparent nonsense in rock era songs can be slightly different. Some groups were a capella and used a 'bassman' singing an imitation of a bass to give the rhythmical backing for the usually high pitched lead singer. A good example is Johnny Cymbal's 'Mr Bassman.'
http://www.lyricsandsongs.com/song/565550.html
(They don't come up with names like that nowadays...)