People should check the error that the gate displays on the LCD panel of the ticket gate. Common errors include "Wrong way up" and "ticket failed" which just mean that the magnetic stripe on that particular ticket is illegible. "No valid route" or "destination exceeded", on the other hand, means that there's a logic error with the gates and/or the ticket.
We have
another thread where people can post issues with the ticket gates (at Heuston or anywhere); if there is a series of issues we will aggregate it and send on to IÉ.