Currently, I am trying to purchase train tickets from Belfast to Dublin. Finding old post that state you need to buy your ticket the day of travel at a train station. Is there anyway to purchase tickets online prior to the day of travel?
Buy on Translink.
Currently there are no trains north of Lisburn, bus transfer service from Belfast to Newry. The line re-opens in the "early autumn" when Belfast Grand Central station opens, The bus station part of Grand Central (replacing the old Europa Bus Station) opens in about a week's time. The rail side is awaiting an opening date.
You should be able to buy the tickets in advance at https://www.translink.co.uk