microsoft authorized resellers in Ireland

I don't think there is one, but the easiest thing to do is to simply purchase from a retailer like Amazon, Tesco, or your local equivalent of target, Best Buy, Walmart etc.

If it's a major chain retailer it's safe to assume they're authorized

Correct, other apps of this type function the same.

I am not certain if they purchase directly from Microsoft or from a reseller. But any reseller would certainly be authorized by Microsoft to be a reseller. There is nothing illegal in these setups

Those are going to be the list of the larger retailers. There may be game stores that are more local in your area that are authorized resellers that won't be on the list. An authorized reseller would get their products directly from Microsoft. If you buy an illegitimate code through an unauthorized reseller, you can be banned.

You'll have to take it up with that site. They are not an authorized reseller and are known for being scammy. You should only purchase from authorized resellers .

Yes you can. Xbox.com will link you to local retailers that are authorized Xbox/Microsoft resellers.