Two options
1. Empty rows of seats one at a time. Put people with bookings in the front row.
2. Why not set up the waiting area in the order as the carriages?
I don't know how many seats are in the waiting area or if the carriages are numbered, but if the carriages are numbered such that 1 is at the Belfast end and 7 is at the Dublin end, set up the waiting area similarly (they will need 7 laminated A4 pages with the numbers 1-7 and some Blu-tack).
For bookings let all the 1s out, then the 2s and so on. Then let large groups out and then let the people without bookings out. Let first / premier out any time they want to. The only problem is that all the 1s would arrive at carriage 1 at the same time and so on. The carriages can be serviced in the order 1-7.
The last thing you want is all the 7s going first and obstructing the platform for everyone else.
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