Jerry, I've never ever heard Rule Britannia sung in a UK folk club. If I had, I would have walked out immediately. Regarding Irish rebel songs, I sang some with a band in the early 70s ( Wearing of the Green, Kevin Barry, Kelly the Boy from Killanne etc) and they were quite common but as the troubles progressed they became almost taboo, though still popular in places such as Coventry. However, there have been a number of songs written since that time that addressed the Irish situation without being explicitly partisan or jingoistic eg Song for Ireland and The Soldier.