There is no evidence whatever that McCaffery was ever illegal.
Paul Smith wrote an excellent long article on it in the 'sadly defunct' folklore magazine from Leeds University (Folk Notes?) where he discusses the issue and said he could find no evidence whatever.
I don't know how accurate it is, but Lomax claimed that the Genoese form of choral singing Tralaleri developed from the time of the Garabaldi uprisings when it was illegal to sing revolutionary songs.
Genoese dockers would sit in groups outside coffee houses and la-la the tunes in defiance of the ban. Eventually, like mouth-music, it became an art form in itself.
Jim Carroll