What are the regulations about changing a web ticket? The website is a bit vague about that.
So are the ticket clerks. Officially the rule is you will be excessed up to half the anytime return fare for the journey (with a zero excess if you've already paid more). In practice, at main stations (Dublin/Cork) you will be charged something (if you're unlucky you may get charged up to the anytime single); at other stations you will just be told to board anyway. (In this case get the clerk to endorse your ticket with the station stamp so that you don't get awkward questions from the gripper.)
All this applies before boarding. Trying to excess on board is not recommended and is likely to get you a 100 penalty fare.
Strictly speaking there is a prohibition on stopping short if the fare for the shorter journey is greater than the fare you paid. Strictly speaking there is also no break of journey on IÉ web fares. You would need to be incredibly unlucky to find a staff member that would enforce the stopping short restriction.
They claim that there is at least one seat at the cheapest quota on every train; I can't say how correct that is but they do sell web fares up until 90 minutes before each train leaves its first departure point.
--- old post above --- --- new post below ---
There is also a transaction fee and a fee for not paying by Laser card. I buy quite a few of these tickets and have accumulated quite a lot in credit card fees and was wondering whether it is possible to get an Irish bank account even though I live in the UK so I could get a Laser card.
Visa debit cards also avoid the extra 1 fee.
This transaction fee means you should not buy open tickets online because you can pay for them at a station by card with no surcharge.
Note that Irish Rail does not accept Amex.