80 class normally works:
06:25 Belfast Central > Whitehead
07:30 Whitehead > Central
08:20 Central > Whitehead
09:00 Whitehead > Central
09:42 Central > Larne Harbour
10:57 Harbour > Central
12:12 Central > Carrickfergus
13:00 Carrickfergus > Central
13:42 Central > Harbour
14:57 Harbour > Central
16:12 Central > Larne Town
17:15 Town > Central
Failing this the other possible diagram is:
06:57 Great Victoria Street > Whitehead
07:56 Whitehead > Central
08:42 Central > Harbour
09:57 Harbour > Central
15:30 York Road > Carrickfergus ECS
16:00 Carrickfergus > Central
16:42 Central > Town
17:43 Town > Central
18:50 Central > Town
19:50 Town > Central.
Both of these are M-F only, and the first one is much more likely and has been the regular turn since last Autumn. Prior to that it worked the peak time only one regularly. It never works diagrams that either start or finish at Larne.
If your boat gets in at 17:00, your only hope for an 80 class that day is to take the 17:47 450 class north from Yorkgate (much nearer docks than Central) to Greenisland for a plus two into the 17:15 Larne Town > Central.
On the second day you should be cool for almost the whole diagram. I wouldn't risk an arrival back at Yorkgate at 18:18 on the 17:15 ex-Larne for a 19:00 boat, but you can bail from the 16:12 at either Carrickfergus or Downshire for a move back to Yorkgate. Leaping at Whitehead (passing place on single line) is a move to oblivion so do not risk it. It looks good in the book, but doesn't work for Belfast > Whitehead > Belfast moves. The train from Larne is let in first, and departs as soon as the train from Belfast arrives.
If you want the timetables and more detail on NIR DEMU ops PM me and I'll post them to you.
Is this likely to still be the case in 2/ 3 weeks time?