Not exactly like that. It's a bit cumbersome for companies in the UK when dealing with NI and vice versa, but it is an open border. So if goods from GB go to NI, EU taxes have to be paid. If the goods stay in the NI, then the company gets a tax refund, and if the goods move to the EU, then nothing happens. Basically NI has to follow EU trade rules if it has to continue dealing with ROI as they are currently. The only advantage is NI benefits from trade deals the UK signs with others. So NI can trade with other countries just like GB.
When it comes to immigration, the full text is not out yet, so it's not completely clear. But I think UK's anti-immigration policy extends to NI also.
I wouldn't worry about immigration though. Once the realities of low-cost labour hit home, the Brits will regret their decision when they see that there are no employees in critical sectors. They will then begin accepting immigrants from anywhere they can, which includes South Asia.