I don't know about the onion Johnnys but Travellers, wherever they were from, relied heavily on being socially integrated, so this must have involved becoming familiar with the language of their hosts
As afr as music goes, we know there were 'German Musicianers' and Italian Organ Grinders on the streets of the cities.
In rural areas agriculture depended to an extent on casual itinerant labour in the form of fruit and potato pickers - the traffic between Ireland and Scotland exchanged large numbers of songs - were told last week of a major Irish traditional singer instructing her daughters going to England to "Bring me back a song"
Once you take on the Norther Scotland/Scandinavian links and the Borders Italian Renaissance influences, you have your direct path to many of our big Ballads
The attempts to nationalise these ballads is a barrier to our understanding them.

As far as singing venues are concerned, Sam Larener once told MacColl and Seeger, Sure, we sand down in the Fisherman's Return' but the real singing to place at home or at sea.
Jim