Over the last couple of years, I have travelled from London to Belfast overland several times, and in several different ways. Herewith a summary of the various options. Note that timings are from London Euston - Belfast/Larne Port; I believe there's a free bus from Belfast Port to Belfast city centre, and from Larne there's a train. Both will take about an hour to reach the centre of Belfast.
Route: Train to Holyhead, ferry then bus to Dublin, train to Belfast
Cost (single): £42 RailSail
Time: 13h30 (overnight)
Epicness: High. Lots of waiting around in Holyhead. On way out: sleep on nice-ish sofas on ferry & fairly comfortable seats on second train. On way back: hideous 4 hours on Holyhead station floor. NEVER AGAIN.
Notes: Daytime route might feel less epic (no need to attempt sleep) but probably even more boring. There is a pub near the station at Holyhead which was open till 2am last Thursday & had a 50p pool table. It also had karaoke in the other bar & someone throwing up in the Ladies, so, yes. Holyhead not the classiest of locations.
Route: Train to Stranraer, cycle*** to Cairnryan, ferry to Larne
Cost (single): ~£60??
Epicness: Moderately epic. Very, very early (0539) start from Euston when we did it outbound, as that was the cheapest train available by a long way. Unsure of timing inbound; possibly also quite epic. Fair amount of waiting around.
Notes: Not sure they take foot passengers (also, 6mi from Stranraer station, though see below re Cairnryan as replacement for Stranraer from Nov 2011). Nice bike ride to Cairnryan; cafes are available in Stranraer while waiting around.
Route: Train to Ayr, coach to Cairnryan, ferry to Larne. Alternatively, could cycle to Cairnryan as with the Cairnryan/Larne option, but I'm not sure how that would work with the RailSail tickets.
Cost (single): ~£50 daytime RailSail, upwards of that overnight**
Time: Outbound : ~14-18h (overnight); 12h (daytime)
Inbound: 16h30 (overnight); 10h30 (daytime)
Epicness: Going overnight from London, if you're lucky you should make the 07:30 train out from Glasgow (sleeper arrives 07:20). Otherwise the next train is 11:42 (tickets would still be valid), so that's a lot of waiting around. Left luggage costs £5 per item, if you want to spend the spare 4 hrs exploring Glasgow. Either way, there's a 1h30 wait in Cairnryan. Daytime, timings are either Euston 05:39 - Belfast 17:45, or Euston 09:30 - Belfast 21:45.
Coming back, overnight there's a 2h20 layover in Glasgow (opportunity for dinner!). Daytime, timings are Belfast 07:30 - Euston 18:00, or Belfast 11:30 - Euston 22:22. Everything matches up well until reaching Glasgow (where at least there are more entertainment options than in Cairnryan, if you are waiting around).
Notes: There is nothing at all in Cairnryan other than the ferry terminal, at least the last time I was there; take sandwiches, unless they've improved the ferry terminal catering during their upgrade. If coming back overnight, I would recommend dinner at Bella Italia on Hope St in Glasgow, which is conveniently close to the station. (Might also be good for lunch on the daytime option.)
Sadly NO LONGER AVAILABLE from late Nov 2011
Route: Train to Stranraer (change at Glasgow), ferry to Belfast
Cost (single): £46 daytime RailSail, >£50 overnight**
Time: 12h daytime, 16h overnight (but mostly on sleeper)
Epicness: 0539 start from Euston daytime on way out; otherwise all matches up well, pretty non-epic. Overnight coming back beautifully smooth. Trains overnight going out appear not to match very well. Ferry actually quite nice, ditto Belfast Port.
Notes: Technically haven't actually done this one outbound in the daytime, but Cairnryan route is the same trains. There's an hour layover in Glasgow for the daytime option, or 4 hrs if taking the sleeper.
Coming back on the sleeper is lovely. Ferry & train matched up beautifully; sleeper train a genuinely pleasant experience involving whisky in the lounge car.
Route: Train to Troon (change at Glasgow), ferry to Larne
Cost (single): ??
Time: 9h returning
Epicness: Trains don't join up with ferry v well, and very early start. Otherwise civilised.
Notes: Haven't done this Troon-Larne, only Larne-Troon. Only runs in the summer. No foot passengers, I think, so you'd have to cycle.
Another option! Reviewed here (summary: now the best bet IMO but more expensive).
* Endpoint for these is Larne, not Belfast. Train from Larne to Belfast Central is I think about an hour; they're not wildly frequent.
** £27 Glasgow-Belfast (RailSail). Then you need to buy the London-Glasgow leg, which if taking the Caledonian Sleeper is more complicated. IN THEORY you can get £19 bargain berth overnight sleeper tickets; in practice it's nearly impossible, although if you can be flexible about the dates, you can get £39-£49 singles. From £53.50 for an advance sleeper (£88 standard non-advance); from £25 for advance seated sleeper (£51.50 standard non-advance).
Note that bike helmets are now a legal requirement in Northern Ireland, which may affect your willingness to take your bike over there. It certainly puts me off. Update: this didn't make it into law; you're still free to cycle in NI with or without a helmet as you prefer. (Thanks to commenter below for correction.)
I think if neither time nor money were a factor, my preference would be to go via Stranraer, or as of next month, Cairnryan (to Belfast) both ways. Probably in the daytime on the way there (though last time we went overnight), and definitely overnight on the way back.
If time is a factor and money is available, overnight out via Holyhead and back via Stranraer is a reasonable compromise between cost/comfort/waiting around. It avoids taking a day of leave just to travel (even with the overnight Stranraer/Cairnryan option, you won't reach Belfast until late afternoon/early evening), and avoids the hideous overnight return via Holyhead; and it saves money on the outbound less-hideous Holyhead trip. However, you won't get much sleep, and what you do get will be on sofas on the ferry or on the Dublin/Belfast train. The overnight Stranraer/Cairnryan option with the sleeper is much more comfortable.
If cash-poor but time-rich, daytime both ways via Stranraer (Cairnryan as of Nov 2011) is only £4 each way more expensive than Holyhead and way, way nicer, especially as the very early start from Euston is not obligatory. The Larne option was OK but a bit faffy.