I'm back from my trip to Ireland.
In the end I came to the conclusion that it was valid to have an overnight stop on the sailrail ticket. It is possible to get the national rail journey planner to give an itinerary that starts before the earlier departure time using the advanced search and allowing an extra 2 hours for connections and travelling via Sandwell & Dudley. I also looked up the ticket restrictions which are no break of journey except for connections purposes. Finally I found the following condition in part 16 of the conditions of carriage about break of journey
(ii) stay in overnight accommodation when you cannot reasonably complete your journey within one day
My ticket was only checked once on the trains and it was accepted without any questions.
I hope my experience is useful for someone else in future.