Matt
You are right that it is hard to choose between the top 5 in Ireland or Scotland:
Ireland: County Down, Portrush, Ballybunion, Portmarnock, Lahinch
Scotland: Muirfield, Carnoustie, Turnberry, Dornoch, Old Course
...and also right that Ireland drops off much more rapidly after these 5 than does Scotland. If they take the Glasgow-->Aberdeen option, they can play on the direct route between the two:
Prestwick
Western Gailes
Troon
Carnoustie
plus a lot of other lesser known gems if they are so inclined
In Aberdeen they should not forget Murcar, which is as good as Royal Aberdeen or Cruden Bay.
Then, if they want to stray further, it is a ~3 hour drive to Dornoch , and the very interesting Castle Stuart along the way.
Golf in Ireland is fantastic, but not for these guys under these circumstances.
Slainte
Rich