Carry your passport around as ID for alcohol, a foreign drivers license etc won't cut it. But for the love of all that is good do not lose your passport. Because if you lose it, it's by by visa and you have to do the visa process all over again.

Assuming you're working here, you'll need to show your social security card (or at least the number) on your first day of work. Get this at the Social Security office, but do yourself a favour and show up about 15min before the doors open, unless you want a loooong wait (standard bureaucracy).

If you're intending on being in the US for a while, then start building your credit score immediately. Everything you've done back in Ireland counts for diddly squat here, so a) open a local bank account, b) get a secured credit card (you're unlikely to be eligible for anything else, and each inquiry can ding your score).

People pay for pretty much everything with debit or credit cards here. I mean EVERYTHING. I'm used to vendors back home not taking credit card for small purchases, but here it's basically expected.

The housing market here is possibly more competitive than anything you've ever experienced. It's not the worst in the world, but as a foreigner with no credit score, and having to stump up up to 4 months rent upfront via cheque (first + deposit + agent fee + sometimes last) can be painful. Room shares (aka, housemates, not literal room shares) can be more flexible, as can agents at the higher end of the housing market.

Again, assuming you're at your own place, then do realize that the likes of Comcast, phone companies etc, will often require a copy of your passport in order to open an account with them. It's annoying. But keep a scanned copy somewhere you have easy access to.