I did the 'daytime ferry' once, as a test, it seemed like a good idea - never again - the overnight one is much more civilised - dumps you into Dublin at 6am and you're going out before many are even trying to get in. Although, the trek to Bantry in one go may be a bit much then. Traffic to Holyhead on the Big Island is also much pleasanter for the 02:40 boat, as well. I had three separate exploded caravans on the way when I sampled the day boat.
I use the 08:05 to return - an early start, but four hours kip on the boat before being dropped into Wales at midday.
You do now have the option of the 'intermediate ferry' on most days, operating about half-way between the main boats.
If you are going to do the Big One, then liaise and we might be able to organise something. I'm over there four or five times a year these days..