Online terms and conditions
 

Tickets booked online (which are reserved seats) have the following information supplied.

Now. I've done this a couple of times recently. But here are my two questions:

1 - what does 'similar' mean in 'as long as the journey is similar'?
2 - what is the supplement fare (i.e. how much), when does it apply, and where could a customer find out answers to how much and when?

Supplement may apply if you have a first class ticket and try to travel on a citygold service

The interesting thing is the rules are all over the place

Imagine you in person book at Abbey Street, they use the same web interface, same ticket, same price in most cases but its not online so do the normal terms apply who knows? You can't tell the difference from the ticket

I would understand a similar journey to be the same journey as booked or one which is a subsection of the booked journey and costs the same or less. (Note that's completely unofficial.)

As for the supplement, it appears to be at IÉ's discretion/whim. Some friends from the Netherlands flew to Dublin and decided they'd be fine arriving into Dublin Airport at 14:40 and making Connolly for the 15:20 to Belfast. Needless to say that was a non-starter and they were charged €5 each at Connolly to revalidate their tickets for the 16:50. On the other hand, someone else who was supposed to be travelling with them (booked separately) and missed his flight showed up the next day and wasn't charged anything.

A charge might be justifiable if you booked an off-peak day return for €44 from Cork to Dublin and turned up wanting to get onto the 08:30 or some other service which would properly cost €61.

A little optimistic, even if the train passed through the airport. :)

All the main airports in the Netherlands have their own train stations :) Of all the airports in the Republic of Ireland only Kerry airport has any connecting rail service to speak of.