I don't see why Cork-Belfast wouldn't work.
The service wouldn't only be for Cork to Belfast passengers, for example people travelling Kildare -Drogheda or Dundalk-Limerick can avail of the service as well as the Cork-Dublin and Dublin Belfast passengers who will also use it.
Most Long Distance trains I use (outside Ireland) carry very few passengers from Terminus to Terminus but still provide a valuable service with High usage as there is a high turnover of passengers travelling shorter legs.
While were at it what about a night train Cork-Dublin-Belfast and vice-versa during months with no R's
