Duty free Macallan 18 from Ireland
I presume you live in the U.S. I cannot specifically speak to the pricing of Macallan18 in the Irish airport duty-free shops vs.what you might pay for it in the U.S. I travel to Britain twice a year and am always surprised at how much more expensive whisky is in Britain (even in the duty-free shops) than it is in the U.S.
For example, I can buy Glenfiddich 12-year old in the U.S. for under $25. In Britain, it sells for over 25 pounds, which is 60% more in $-terms.
I would check the price of Macallan 18 in your nearest BevMo or big volume store (Costco?) before you leave for Ireland. I would be surprised if you can buy it cheaper in Ireland duty-free.