In some cases yes, 'derry down' usually meant the higthly popular 'King John and the Abbot' tune and chorus. 'Bow wow wow', would have indicated the tune and chorus, both of these immensely popular in the 18thc. However they seem to have been just a rough indication of a chorus in some cases, or even a decoration.

Your example of 'fol de rol &c.' could have been several choruses/tunes without further information. It is possible that some ballads were simply being reprinted and were already well-known, and therefore to both street seller and buyer the chorus would have been obvious.

Fol de rol de dido, fol de riddle day.
'Fol de rol de rol de dol', in various combinations
Fol de rol de rol de ray

A 'fol-de-rol' was in the 18thc a toy sold at a fair, probably named after the chorus.