I think it's €2.60, no change is given and you must pay in coins only, even a €5 note is not allowed. However unless you are not taking any further public transport during your time in Dublin, you should just get a Leap Visitor card at the airport then you can take whatever regular buses, trams and commuter trains you like.
The Leap Visitor used to be valid on the dedicated 747 airport bus but this route was cancelled because of covid.
Thanks for that, it was perfect timing as I read it just after arrival at DUB. 👍
Yes, 2.60 EUR, and thanks to you message I had the correct coins ready. 🙂
It's only an overnighter, so I didn't bother with the Leap card.