We looked into this when there was a possibility our favourite rugby team might be playing a big match in Edinburgh - unfortunately the French won!! However, we reckoned Larne - Stranraer was the best. Take the train to Belfast and you should be able to get the bus to Larne. The Dublin Belfast Train trip is actually a wonderful trip and would be worth doing even just for the ride itself.
Thanks beanalainn. This post and ones to Best Place to pick up rental car have been really helpful. There seem to be direct buses from Dublin airport to Belfast so may take one for this but during previous 4 day stay in Dublin planning to spend a day seeing Belfast so will consider train. Debating whether to do day on our own or take a day bus tour to Belfast but train sounds more relaxing.
Hi Avoca - the train is really lovely and relaxing and you can even get a glass of wine on board. I don't know about the Guiness though. But certainly take the train - the track even goes over sea inlets etc. I would prefer it to road.