Questions About Belfast to Dublin

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The wife and I plan to travel from Belfast to Dublin via train on Saturday March 10. We were hoping to go fairly early so that we would have more time in Dublin. However, as I look online, many of the fares are already sold out. Here are my only train options that I can find:

2:10-4:10 route for £17.98 for two (WebFare) or £56.00 (Standard class)
or
8:00-10:00, 10:35-12:35, 2:10-4:10 routes are all available in First Class for £88 for two

The jump from £17.98 to £88 is significant, but it would allow us to leave earlier.

Here is part of the problem: when we booked our flight, the only option was with 1 layover in Newark and it is only 1 hour. Most of the time, flights are on time, but with only an hour, I am somewhat concerned about missing the connection and therefore arriving late. If that is the case, I would probably lose my pre-booked train tickets. No big deal with £18, but £88 is kind of a big deal. Or am I worried for nothing?

I know that buying WebFare is much cheaper than buying in person, but is First Class that way? Should I just wait until we arrive in Belfast to buy the tickets if we are going first class? Will it be sold out? I know that there are other options like buses, but we have never been on a train so we are leaning towards that if possible.

Any help is appreciated.