Flowman, one way or another I hope that you have a great trip! Certainly the Antrim Coast Road has to be one of the finest roads in the British Isles.
In relation to the England-IOM-N.Ireland route it is not publicised by the
Isle of Man Steam Packet Company. I reckon that they are crazy for not doing so. The Island is excellent for a stopover of say 3 or 4 days (more of course!!!). For motoring enthusiasts it has the TT course which in some parts does not have a speed limit (but watch the bends and the walls) and a lot of other fine roads. It also has a number of very interesting national heritage sites, some good museums, wonderful coastal scenery and glens and lots of paths plus 365 days of sunshine p.a. (hmmmmmm). The landscape for such a small place (35 miles X 15 miles) is very diverse. If you like old trains we also have a 19th century steam train that runs in the summer over about 15 miles and a 100 year old electric tramway that goes a similar distance.
On the ferries as I mentioned you would need to talk with the IOMSPC directly to get the proper fare - using their one way fares on the website comes up with horrendous costs. I would recommend that you use the website to check what a return fare to say Heysham would be on the days you wanted and then call them to ask for the through ticket - also don't let them charge you for not booking on-line - just say that their system doesn't offer the route so you have to do it person to person.
Overall this would mean that you have to pay for 3 ferry trips not 2 as you would still need to get back from NI - but if you have the time and fancy a bit of a different experience its worth thinking about.
PS: the ferry sailings to Ireland are only available from Spring to September.
