Yes, you need this special one instead of the normal ID.
It's €7. I literally went up to the desk (the ticket desk - not the info desk) in Heuston (so the length of time depends on the queue), handed in my passport photo, wrote my name and address on the card, they stuck the contact plastic over it and I bought my Galway weekender ticket. €17.
And another one today. Saving me a fortune!
Although Mark has some posts on the board that sometimes there can sometimes be other cheaper tickets (depending on the destination?), so check that out.