I'm no expert on the subject of Sail-Rail fares, but the best route would be via Holyhead and Dublin if coming from London. Virgin Trains run one service per day on weekdays from Euston to Holyhead, otherwise you'll need to change trains. I have no ideas as to times. Please note you will need to book in advance and obtain reservations as these are compulsory, at least for the ship part of the journey. Since Arriva Trains Wales set the fares, they will likely be the most knowledgeable ones (knowledge of Sail-Rail fares and booking procedures are poor among some rail staff, though in theory Sail-Rail tickets can be bought from any staffed station and I suspect the knowledge at London Terminals shouldn't be too bad. Euston would be the most relevant London terminal should you wish to take this route). The best bet would be to give telesales a ring on 08709 000 773 or consult the following website:
http://www.arrivatrainswales.co.uk/SailRail/ In this case, I might be tempted to go with telesales, as they may be able to advise you better as to your options.
There is then included an onward connection via rail from Dublin to Belfast. These tickets are very good value, starting at £49 per person for a single.
http://brfares.com has the available fares (maybe a few months out of date), I don't seem to be able to get it to apply a disabled adult discount though.
There is an alternative route which is to go via Ayr and Cairnryan (bus is provided between the two places), and again one ticket covers the whole journey and I believe you need to book reservations in advance. Again, the sail-rail website should help you out here and recommend should you want to book over the phone the ATW telesales number above. Once again, tickets are from £49 each way per person. Note that here the ferry goes straight to Belfast.
Beyond my advice, I would recommend reading what
The man in Seat 61 has to say, just don't book your tickets through there as it uses Raileasy which charges unnecessary booking fees.
As for times and routes, the National Rail Enquiries Website also gives itineraries but does not sell tickets (
www.nationalrail.co.uk)