RunningMax, I've only done 2 ultras: the Mourne 52 miler and the Causeway 39 miler. I'm slow as I walk quite a bit, but I'm not proud and just love being out on the trails. I did a half marathon trail race between Morzine & Avoriaz a couple of years ago and would love to do a longer one sometime in the Alps.
Runningmax - there are many opportunities for your husband to catch up with you along the route. This is a tourism area with car parks at a number of points along the way. It is off season so the roads will be quiet. The finish in Portballintrae is a busy place that day and he may have to park quite a distance away in the town. Ah... my wife for a long time just thought I was daft getting into endurance stuff. She had a point considering the number of silly running injuries I've picked up along the way. For example, last year I sneaked down to the Mournes for a 'run', but had to come clean with her when I returned at the end of the day with a 52 mie ultra t-shirt. She appreciates much better now what it means to me and is very supportive. I can't let it take over though, my family come first. If you can both get down to the Mournes while you're over, you'll see another beautiful part of our country.
The Causeway Coast marathon is out & back, ie. Portballintrae to Carrick-a-Rede car park and back. There are a few videos of it on YouTube. As for spectators, we had a herd of cattle on the beach this year. Otherwise, it's just us, the trail and the amazing views. This is Northern Ireland at its best,
Dublin is awesome. It has incredible spectator support. Just read up about it on the comments in the Events section of the RW forum or in this thread.
I've never done the Mooathon but hear it is great fun although tough.
I haven't done the 50k, only the 25k. I have done though the Causeway marathon & the 39 mile ultra. The hills are tough but you'll be fine with the 50k if you pace yourself. I don't race these distances and I'm happy to just stay within the cut off times. A marathon training programme would be ok, but suggest also you work on leg strength & hip flexibility to prepare you for the hills. The steps down in the trail can feel even harder than the ascents so training on stairs will prepare you well. Try taking sometimes 2 steps at a time. Do you work in a high rise building? Spin classes & circuits with squats & lunges help a lot.
You don't have to commit yourself this early. How about doing the training and then see how ready you are. You'll find the weather a big factor as it is so exposed in parts. If it turns out to be a nice day, there is no better place.