Manhattan is expensive; the further you go from Manhattan, the lower the rent. I live in Hoboken, right across the river in New Jersey. New Jersey carries a moderate stigma (snotty Manhattanites won't travel there) so you end up getting more for your rent. I wouldn't overlook it if you're trying to save money, as long as you're walking distance to a PATH station. (The PATH train runs 24 hours, just like the NYC subway, except it's slightly cheaper and cleaner.) I used to live on 138th St. in Manhattan, and now I have the same amount of space in a nicer neighborhood for less money - and my commute to the relevant parts of Manhattan is both shorter and more pleasant.
The tech community's there if you want it, but it's not omnipresent like it is in San Francisco. If you put in an effort to meet people you'll be fine. I wouldn't describe it as 'more cut-throat' at all. People are both nice and social.
Recruiting is brutal. Then again, I hear recruiting in San Francisco is also brutal.
Health insurance is a pain. If you've got real funding, a professional employer organization (PEO) like Ambrose is the way to go - they take care of payroll and health insurance. If not, talk to a broker and get something with a high-deductible - you're getting it in case of catastrophe, not for every sniffle. Don't go without.
I'm an immigrant, but I had my permanent residency sorted long before I started starting companies, so I can't help much there. I can tell you that as a Canadian, I found U.S. Citizenship and Immigration Services inefficient and irritating to deal with, but not malevolent. They'd probably have a similar attitude towards you - Ireland's a first-world country. Just be completely honest and make sure your paperwork is perfect.
If you do relocate, send me an email, and we'll get lunch.