Not sure if I will make this (depends if I have a MX5 then!
), but
Accomodation; can heartily recommend Ballinderry Country House:
http://www.ballinderrycountryhouse.co.uk/
The owner works for BMW and is a Triumph enthusiast. A few of us stayed there in 2005, after travelling over for the Dundrod TT, so they know MX5s there. Country location, but close to Belfast. Loads of parking, nice facilities, great Ulster fry. Stormont and the Ulster Transport Museum are both accessible without actually going into Belfast itself. About 15 minutes drive from either of the main Belfast ferry points.
Ferry; 6 years in the province, and I ended up preferring the overnight Liverpool-Belfast ferry (Norfolk Line).
And an answer to a silly question I once had; no, you do not need special insurance to visit Northern Ireland.