Train between Belfast and Dublin Oct 24

You can't buy now, as no-one can buy tickets in that direction until 28 days out, so 26 or 27 September in this case.

There is no such thing as a better rate, the rates are non dynamic rate, purchaseable only from Translink.

£14.99 Enterprise Low, fixed train, non refundable, £15.99, Enterprise Semi Flexible £15.99, non refundable, Buy online up to 80 minutes before your train departs from Belfast. Amend your booking up to 24 hours before that departure time.
Enterprise Flexible £16.99- Buy online up to 80 minutes before your train departs from Belfast. Amendments or cancellations can be made up to 80 minutes before that departure time.
Enterprise First [Class] £48. (conditions as the Flexible fare above).

Normally the 0700, 0800, 1300, 1400 and 1900 are Enterprise Train sets, all but the 1900 have a full catering service.
The 0900, 1000, 1100, 1500, 1600, 1700 and 2100 use Irish Rail Inter City trains - some with a trolley for catering, the 1000 and 1100 have full catering
The 0600, 1200 and 1800 use Northern Ireland Railways trains with no first class, no seat reservations, and only trolley catering.

However note that there is rail engineering work that day between Malahide and Dundalk, so only a 2 hourly service will run- on the even hours- 0600, 0800, 1000 etc.
As it is also an Irish Bank Holiday weekend trains will be busy, so book as soon after 27 September as possible. Tickets can't sell out, but you may not get a seat waiting until you are in Belfast.

Thanks for the information, this really helps. I see translink has a bus running from Belfast to Dublin. What is your opinion on taking the bus? With the engineering work and the holiday you mentioned, would bus be an alternative?
I dont want to consider bus unless it is direct.

This spring, we were staying at the Europa hotel in Belfast. After breakfast, we walked with our luggage a short way to the train/bus station and took a pleasant bus ride to the first stop, Dublin airport. The second stop was in Dublin. We did have reservations, but when we went, the bus was maybe 3/4 full.

Bus is always an alternative (and cheaper), and Translink is the cheapest of the three available direct bus options, although the one least used on this forum, as the other companies have marketing budgets. Translink is the state operator in the north (running all city, rural and long distance buses) and the original Belfast to Dublin operator, so doesn't feel the need to market itself in the same way.

Like the train it can't be booked more than 28 days out, and most locals will book it pretty much day of travel, not far in advance.

No toilets on board the bus, functional and gets the job of travel from A to B done perfectly well- that applies to all the 3 bus lines.

Thanks for the information!