Booking an Irish Rail ticket - can someone explain these “extra” add-ons?

Booking a train ticket from Galway back to Dublin on the last day of my trip. I’m trying to find the best way to get from Heuston station to the airport. I think these add ons are what I need, but I’m not totally sure.

There are two additional products I could choose:

1. Return Dublin Express (€10.00)
2. Single Dublin Express (€8.00)

Can someone explain what these extras are, and what the difference between return and single is?

Thank you!