The standards of seating are perfectly adequate in both classes. The main difference is seat width as seats are 2+1 in First Class and 2+2 in standard.

The only real reason to go First is if you want a main meal served to you. As ever on railways in Ireland these are very well done, but the kitchen only serves First Class. In Standard Class there is a takeaway counter which does just beverages, sandwiches, etc.

The other difference is in passenger numbers; not a lot of people go First, and even if you take a table for four you will normally have it to yourself.

Sit on the east side (right going north, left going south) of the train for the views across the sea at points along the way.