No there's no special bus leaving Dublin on Saturday as far as i know, but you could get the Bus Eireann (or Ulsterbus) from Dublin to Belfast and back on Saturday, and judging by the Translink website (N.I. Railways and Ulsterbus/Metro) there are regular buses and trains running between Belfast city centre and Cultra, and i presume will drop you close to Cultra where the event is taking place, as i've never been in Cultra and don't know the area or where the buses and trains drop off/pick up.

Just one question, what time is the event starting at tomorrow and what time will it run until? (approx) And is there a charge?