I've never heard of any logical reason for Fol Di Dols or tura lur ayes, in fact I recall being told by by gran long time ago that they're simply song writers tricks to fill in where they can't think of another line or to fill in little lilts, much in they same way in the 60's when the Beatles added lots of Yeah Yeah's to fill out the tunes, or when Rock, N'Rollers added terms like 'be bop a loola' or 'shebooms' so hey ho diddle eeh aye dum