The trade status of Northern Ireland was one of the major sticking points in the negotiations between the UK and the EU--especially during Theresa May's tenure in No 10. The question was whether border controls would be built between the UK-NI and the EU-Ire.

But, the problem was never that the UK would put up a barrier. The problem was always that the EU was afraid of lower priced good getting in the UK, crossing the Irish border, and getting into the EU through the back door.

The EU hasn't even passed the Brexit deal through their parliament, and they are already invoking a clause in it and putting up that border:

https://www.express.co.uk/news/politics/1390911/Brexit-news-uk-eu-clause-european-commission-vaccine-latest-AstraZeneca-supply