If you look carefully you will see that the machine is likely displaying 'smartcard only' or some other message It does that when the mechanism is jammed.
The key to getting this resolved is to get the exact text which is displayed by the turnstile when it refuses the ticket. From that we can find out if
1. Is it the software in the gate
2. The ticket is unreadable
3. The ticket is encoded wrongly
We have put a fair few ticket scenarios to IE which tripped up the gates and they did get fixed. Irish Rail did issue a batch of annuals with the wrong expiry date a while back. The software was changed and a number of defective readers replaced and from experience the gates in Heuston have a very permissive set of rules compared to elsewhere.
The biggest tip, do not insert the ticket into the validator on the bus, thats a very common point at which the ticket becomes unreadable. If a ticket is refused it was probably damaged by the previous validation.
If you do have a rail only Dublin Short Hop zone ticket, get it replaced with a smartcard for free. There is a proactive program on going to swap these tickets out which has included IE staff showing up at large taxsaver customers to swap the tickets. The smartcard solution is the best as the magstripe option is never going to be 100% robust plus it eliminates the excessive resissue charge following loss/theft.