The public transportation system in Barcelona is extremely comprehensive, effective and cheap. The TMB website is a fantastic resource of timetables and routes. As expected with such a design conscious city, QR codes have been introduced at some bus stops to give up to the minute information directly to your phone. For full information on bus and metro routes visit: www.tmb.cat
Buses A1 {terminal 1} and A2 {terminal 2} run regularly between the airport and Plaza Catalunya. The No.17 Nightbus runs from both terminals and Plaza Catalunya from approx. 10pm-5am. For Girona airport connections take the Barcelona Bus. Full information at: www.aerobusbcn.com