I suspect that a seat number is always at a certain end, certainly on Cork-Dublin, but it may vary occassionally on other trains.
While I suppose that helps you, it might not help everyone else, so yes numbers and arrows might be useful.
<-- 1-36 and 37-72 -->
Another thing might be to point out "This carriage mostly reserved" and "This carriage mostly NOT reserved" or similar wording.
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