I am quite familiar with ferry options, north and south, having lived in N. Ireland for 7 years (and ran the MX5 Club there).
Personally, I prefer the truck ferries, either Liverpool-Belfast, or Dublin-Belfast. Norfolkline serves north and south. I prefer them as there is less driving to do in GB before you get to the ferry; for me, getting to Stranraer or Holyhead was a bit of a haul, compared to a quick dash up the M6 to Liverpool. IOM ferries used to do an excellent Belfast-Heysham fast ferry, but for some reason never reinstated it after their hydrofoil broke down. P&O also do a truck ferry from Fleet to Larne (N. Ireland).
Daytime truck ferries can be quite cheap (espcially tues and thurs), but they are slow boats. I always work out if I can get the overnighter; you get a cabin, dinner and breakfast all included. Overall, cost wise, worked out the same for me as the other routes, if you take petrol into account. Plus you start your holiday refreshed.
Getting out of Dublin at times can be a bit of a nightmare. Dun Laoghaire is a bit easier if you plan on heading South into Wicklow and beyond, as you’re virtually out of the city.
Stranraer-Belfast lands you in the heart of the city; Belfast isn’t very big, so its easy to get out. Liverpool-Belfast gets you to a ferry point just outside of the main city, and is convenient if you’re heading up the Antrim coast, but crossing Belfast, thanks to the West Link. Larne is aways up the Antrim coast. Some fantastic roads in the North; Antrim coast, on a clear sunday, is probably one of the best drives in the world (most of it hugs the foot of a cliff, at sea level). N. ireland also has a strong motorsport tradition; you can still follow the Dundrod TT course, which was the last car road race in the British Isles (Mike Hawthorn holds the lap record); its still used for bike racing, so its cool to see the start line, the old starting tower, and marked corners. Down in Armagh, Slieve Gullion is a fantastic 8 miles of one-way switch-backs. I fear though that the surface has worsened over the years. Generally, however, the road quality in the North is better than the South; it varies a lot down south. The main trunk routes are ok, but it can be a bit difficult off the beaten track. The North is roughly comparable to anywhere in the rest of the UK.
The North is small enough for you to base yourself at one B&B. I organised a club trip in 2005, and we stayed at a superior B&B at Balinderry; very good, and the owner was into his Triumphs. One myth to scotch is the notion you need special car insurance in Northern Ireland. You don’t; I had that question once from someone, who was using at an excuse not to cross the border while on a visit. Don’t visit Northern Ireland in July; stay clear of the place, unless you know it well. There are some spots you don’t want to be during this time. Enthusiastic local MX5 club though will see you right.
Petrol stations down south will generally take Sterling
North or South; I prefer the North for driving. Some great driving down south, but some of it takes a while to get to. You can of course do the Ring of Ireland.
To see up North:
Slieve Gullion
Antrim Coast
Strangford Lough/ Downpatrick
Torr Head
Mournes
Antrim Glen
Rope bridge, Giants Causeway etc
Kirkistown; a local company does regular inexpensive trackdays there
Ards Penninsular
Belfast centre; worth a look.