Chris makes a good point about advances.  I should have made it clear that I was talking about the SailRail standby tickets. 

I find the wording printed on the standby ticket a bit confusing.  The route is given as HOLYHD STENA_S ON DATE SHOWN.  I find that confusing because the date on the ticket and the date on the ferry reservation are different. It also misses the important fact that the ferry leg is a reserved leg and  you must travel on that particular ferry.  Instead it gives the impression that any Stena ferry on that date can be used. 

If you can get a ferry reservation for the 0855 sailing with a journey starting the previous day.  The overnight stop in a hotel does seem to be allowed. 

Thanks to everyone who has contributed to this thread and helped me find my way through some opaque ticket rules

 

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  
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.

Thanks for all the replies.  I can now see why I had difficulty getting a clear answer when asking the ticket staff.  It does seem to be a bit of a grey area as to what the rules allow.  There is one journey offered by the journey planner that involves leaving the station for a connection, the 23:06 departure involves a walk from Moor Street to New Street in Birmingham.  The ^43 fare is available for that service so how does that fit with the break of journey rules?  Would the same logic apply in cases where you can't stay in the station because it is locked?

The morning departure from Oxford does sound like the most pleasant option, however I think my employer might take a dim view of me setting off after breakfast 

If anyone fancies playing the split ticket game the full set of requirements are:
Leave Oxford after 16:46 (normal train home time) on the 27th
Arrive in Malahide (just north of Dublin) by 14:00 on the 28th
A decent nights sleep somewhere, at least 7 hours
I have a season ticket from Oxford to Charlbury so gold card discounts can be used

A second related question.  Is it always possible to buy on board Northern Ireland trains?  I'll be travelling onwards from Malahide to Bangor in the evening of the 28th.  The journey I'm planning on making involves a quick change in Belfast and it doesn't appear to be possible to buy tickets online for the Belfast to Bangor leg.

Thanks for your quick reply Grahame.  It is a bit disappointing that there isn't a way to do the journey with a good night's sleep.  I'll have to pack a sleeping bag and roll mat and find a quiet spot on the station for the night 

Hi everyone

I'm new to the forum and hoping that someone can help me find the best ticket for my journey. 

I would like to travel from Oxford to Dublin after work on Friday the 27th of May.  I don't have to be in Dublin until lunchtime on the 28th.  The national rail journey planner lists two potential journeys leaving at 18:38 and 22:30 and arriving at 05:45 and 12:10 respectively.  The earlier journey involves catching the 02:30 ferry and the later journey has a five hour wait overnight at Birmingham New Street, neither of which I'm particularly keen on. 

What I would like to do is start on the earlier departure as far as Holyhead stay in a B&B overnight and then catch the later ferry on Saturday morning.  Both journeys have the same ticket type available, a ^43 off-peak with route HOLYHD STENA SHP, but no mention of travelling on a particular train (or ferry). 

I have tried asking if my journey would be valid with that ticket at Oxford station and Arriva trains Wales phoneline.  Oxford station thought I would have to go on the exact trains offered by the journey planner whereas Arriva thought that any departure from Oxford on the 27th would be fine with any ferry on the 28th.  Last time I used the train and ferry to Ireland (in 2010) the ticket allowed travel on any train but the ferry leg had to be reserved. 

Can anyone point me at some ticket terms and conditions that would clarify if what I want to do is valid and if so which ticket should I buy?

Thanks for your help

Steven