For the
Antrim trip I suggest that you look at the DFDS (formerly Norfolk Lines) ferry from "Liverpool" to Belfast at
http://www.norfolkline.com/EN/Irish_Sea_Ferries/Belfast_Liverpool/. An 8-hour overnight crossing and so longer than the Stranraer crossings, but it does allow you to get a night's sleep.
I say "Liverpool" because, rather like Ryanair, what they actually mean is Birkenhead. The 12 Quays Terminal is miles from anywhere. If you've got serious luggage, then I'd do a taxi from Liverpool Lime St through the tunnel rather than faff about with MerseyRail's local train (and another taxi on the Birkenhead side).
I've travelled with them on their Dublin - Birkenhead route. The accommodation is fine. At one stage they used to offer an inclusive price for BB&EM, but they seem to be charging for food now. Not that the food's that wonderful - this crossing is mainly for lorries and the offering reflects this (which does make for a big breakfast).
On the Dublin route they won't let you walk onto the ferry on your own. Instead you're driven on in a people-carrier. That's fine in the evening, but in the morning they have to have moved quite a lot of the lorries before the people-carrier can come and collect you. You just have to be patient.
And on the Dublin route, you arrive rather early. I haven't looked at what time it gets into Belfast. Since you need to be in Antrim at 08:45, that may not be such an issue. But I can tell you that Birkenhead at 06:00 leaves something to be desired - though the taxi will turn up promptly and whisk you off to Lime St, not having anything else to do at that hour.
Cheaper offerings on the Irish Sea ferries are to be found at
http://www.sailrail.co.uk/, where you should find that you can look for a Melksham - Antrim through ticket. But for some mad reason, you can't book it - that has to be done at a railway ticket office. But those cheap fares only work on the short crossings - Fishguard, Holyhead and Stranraer.
Meanwhile, for
Frankfurt, I'd look at the Harwich - Hook route operated by Stena, at
http://www.stenaline.co.uk/ferry/routes/. Big ferries. Overnight crossings. Not just lorries, so a 'proper' restaurant. And a simple train-to-ship walk transfer at each end - though these do involve long ramps, so I hope your luggage has wheels. Trains from Hook to Rotterdam every 30 minutes.
Stena have a cheap train-ship-train ticket so that you can travel from anywhere in East Anglia (including Liverpool St) to anywhere in the Netherlands for ^34. See
http://www.stenaline.co.uk/ferry/rail-and-sail/holland/. Then you'd only need a ticket from the Dutch frontier station (Venlo?) to Frankfurt.