What you can do is the following:
Find someone you trust with a valid home address in Chicago/NY. Order the laptop through Amazon (no sales tax) and ship it to their place. Then you have to smuggle it back to Ireland (this is the only illegal part here).
Within the EU, you do not get your VAT back. I was living in switzerland for a while, and we once bought some expensive stuff in italy. When you buy you ask the shop to give you some extra tax documents, and then you can reclaim your tax back in switzerland. At the same time, they collect the swiss sales tax, i.e. I got about 13% back (20% italy tax - 7% switzerland tax), and of course there is a fee involved.
Now, even if it would work the same with the US, the net effect would be that you get your US sales tax back (~10%) but you will have to pay the 20% ireland sales tax.
So the best way is to buy online (not from apple.com!) in the US or buy in a state that has no sales tax, and then not declare it when you return.
I'm not sure what the limits are for Ireland, but for Germany everything above Euro 450 has to be declared, otherwise it's fraud.
I'm surprised that this is being discussed here so openly, while everyone who asks for advice how to unzip/install some pirated software gets flamed and a warning immediately. Double standards I guess.
Edit:
Here is an official statement concerning a sales tax refund:
https://help.cbp.gov/app/answers/de...les-tax-paid-while-visiting-the-united-states
The bottom line is that you have to ask the state. Illinois:
http://tax.illinois.gov/Businesses/TaxInformation/Sales/rot.htm
Excerpt:
Sales — The following list contains some of the most common examples of transactions that are exempt from tax.
[...]
Sales to out-of-state buyers
I have no idea how to make use of the fact that you are exempt from sales tax... you might have to negotiate with the vendor (Apple store)!