Northern Ireland has its own currency that’s considered illegal but people still pay with it.
Pounds sterling in Northern Ireland have been issued since 1929. This currency is legal, but technically, it’s not considered legal tender anywhere (including Northern Ireland itself). Still, they’re widely accepted. In 2006, the country issued 1 million £5 notes depicting Northern Irish football player, George Best. The entire issue was taken by collectors within hours of becoming available in bank branches.