On most ships the price advertised is from a London Airport, and supplements added for regional departures. Watch teletext on page 227 and you will see cruises advertised especially Thomson cruises where they list the prices for regional departures. Orient Lines prices include regional airport departures. I would advise that you should book your regional flight via the cruise line or travel agent rather than doing it yourself, reason being, if you book it yourself and the flight is delayed could miss your connecting flight. If you want to book the flight yourself better to travel the day before.
As far as i know thomson and island cruises prices cover the flight from n.ireland as we had looked at these companies.your travel agent will know exactly which cruises include flights from n.ireland. it would be nice if a cruise company sailed from larne or belfast then we wouldn't have to worry about excess luggage on flights.