[pierrem15]

There will be no hard border on the island of Ireland. So people and goods will flow easily across the island. But there will be a customs check between NI and RotUK

I thought the internal EU customs checks were a redline. Although if the EU court system no longer has any authority in GB, I'm not sure how it will be enforced, and if there is a free trade agreement, I don't see what effect the internal customs checks would have.

[Winniesboy]

The checks will be mainly on regulatory compliance, and mainly on live animals and foodstuffs moving to NI from elsewhere in the UK. That’s because some EU regulations will continue to be observed in NI. The checks will be carried out and enforced by British customs officials.

[Cronos]

There won’t be an internal EU customs check.

There will be an internal UK customs check between NI and the rest of the UK (RotUK)

Northern Ireland will remain judicially and politically part of the UK, but will follow EU trade rules and commercial standards/regulations.

[Winniesboy]

Here's a useful explainer on the N Ireland customs deal. If it sounds like squaring a circle, that's because it is:

Northern Ireland protocol