Why can you get a 16 - 25 discount but not a DIS or SNR? Seems a bit odd.
I am planning to do Belfast then Dublin and come back through Holyhead.
Is there an unlikely railcard discount on the supplement to be had?
What happens if one of your trains is late and you miss the ferry? Assuming the last one has gone go they find you a nice cheap bed for the night?
You
CAN get a senior railcard discount. You just can't get the DSB or FAM one.
The sleeper berth supplement is full price. No discounts are generally permitted on supplements.
With your itinerary, you should get a Winchester-Belfast NI return route Stranraer. This ticket is
not valid via Holyhead.
However, what you can do, providing you do this at the same time as you buy the original ticket, is purchase an "alternative route" excess ticket to change the return portion to route Holyhead. That excess would cost £0.00 (free). That is your cheapest option to go out via Stranraer and return via Holyhead.
Yorkie or some other regulars will probably tell you more about the excessing procedure if you are unsure.
With regards to the late running/missed last connection situation, I believe all these Irish tickets are issued under the CIV Conditions of Carriage, giving you additional rights than the NRCoC ones.
Effectively under these conditions you won't be left stranded, however, say you arrived at Holyhead at 0200 because the ferry was late, you missed the train that left at 0030. If the next train was at 0400, you would have to wait the 2 hours. If the next train was not due "within a reasonable waiting time" (staff discretion), you'd probably be taxied to somewhere like Chester. The railway very rarely, if ever, any longer provide overnight accommodation, it being easier and cheaper to provide a taxi or bus (assuming there's quite a few people).