DART in Dublin

Hi guys, im from Belfast and im coming down to Dublin once again for Megacon/ComicCon, and i saw I can get the DART from Connolly to Sandymount and then walk to RDS. Problem is, I am greatly confused as to how the Dart works. Can I buy tickets online? Do I tap my card somewhere at the station? Do I buy my ticket on the train itself? I am always paranoid about booking tickets/accidentally not paying so I’m sorry for sounding super naive, but just wanted to know how the dart works. Thank you :)