Are you sure you are not overreacting a little bit here? Could you please explain me how is it that it's illegal to buy a product in other country while on vacation?

As far as I know you can spend your money in whatever way you want while visiting another country.
It's perfectly legal to buy anything (okay...almost anything) anywhere you want. However, it
is illegal to make a false declaration to an Immigration, Customs or (in some countries) Quarantine officer. What some people are encouraging the OP to do is to represent the foreign-purchased computer as a domestic-purchased item...in other words, they're suggesting he/she lie to those officers. That piece of paper you fill out on the plane as it's approaching your destination? It's a legal document you sign, not just a friendly survey.
As I've said in an earlier post, there are some categories of people you should never lie to, and Customs & Immigration officers fall into that category - not because I'm a goody-goody, but because they can make your life extremely miserable on future travels. US Customs & Immigration officers for example have the authority to arbitrarily ban visitors from the country for five years on the spot. Naturally, OP is talking about Ireland which I suspect would be a little more easygoing when it comes to bans, but there are other financial and legal penalties the OP could be subjecting themselves to.
He's talking about buying a computer while in the USA, not about going there and steal a computer from an Apple Store. He'll be paying taxes like everyone else does, so, what's the problem? I've even been to places where you can demand to get your taxes reimbursed (although some restrictions apply).
OP is also obliged to pay taxes and/or duties, as required by his local laws, when returning to his country with items obtained elsewhere. The funny thing is that all these people are saying "burn the box" and ignoring the fact that there's a very good chance that even with a full declaration, the OP will just be waived through (and yes, that's "waived").
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I've probably been through 20-30 different customs inspections on the US side and foreign countries - thanks to my travels and military deployments.I do however know people who have brought Apple products from the US to other countries without claiming it.
So, in other words, as a (presumed) member of a nation's Armed Forces, you are saying you endorse and encourage people making false declarations to government officials?
Perhaps during one of your overseas deployments, someone made a false declaration to some soldiers about being on your side as you travel through...say...the hills of Afghanistan, or perhaps says "oh...there definitely won't be any IEDs along this road...". Would that be okay? According to you it would be, as long as they don't get caught.
It can be done it's just a risk to take, worse comes to worse they find out you didn't claim it and you saw oh I forgot or it was a gift.
Worse comes to worse they seize it and fine you. If you're a frequent traveller, you could lose certain privileges or be subject to more detailed inspections in future.
Is it really worth the consequences if you get caught? I don't think so...that's why I'm happy to pay import taxes when I buy items from other countries, because I'm still saving money.